Firefox 145.0a1 Nightly: what’s new, what breaks, what to test first

Nightly isn’t a free upgrade. It’s a lab build that can wreck your weekday.

I’ve watched teams install Nightly for one CSS feature, then lose access to an internal dashboard because the browser suddenly treats the LAN like a threat surface. Firefox 145.0a1 brings real productivity wins, but the “security defaults” matter more than the shiny bullets if you run anything on localhost, a NAS UI, or a printer admin page.

The three changes I’d test before you touch your daily profile

Test these first. Seriously.

If you only have 10 minutes, focus on local network access, graphics stability on macOS, and extension compatibility. Everything else feels optional compared to “can I still reach the tools I need to do my job.” Some folks skip testing for a Nightly point release. I don’t, but I get it.

• Local network requests (still) get blocked by default in Nightly: If a site tries to talk to 127.0.0.1, a home router, a NAS, or a service on your LAN, Nightly can block it unless you grant permission. This started earlier than 145, but it remains the sharp edge that breaks real workflows.
• macOS GPU process enabled by default: Firefox runs WebGPU, WebGL, and WebRender work in a separate GPU process, and Firefox can restart it after fatal errors. If you hit weird rendering glitches or perf drops, you can probably flip layers.gpu-process.enabled to disable it.
• Profile and extension risk: Nightly changes fast. Use a separate profile if you care about your saved sessions and your “I need this one extension for work” setup.

What’s actually new in 145.0a1 (the parts you’ll notice)

Copy Link to Highlight rules.

The thing nobody mentions is that “copy link” features only feel good when they land exactly where your reader expects. Firefox now lets you select text on a page, right-click, and copy a link that points to that exact passage using text fragments. I’ve had this fail on pages with heavy client-side rendering where the text shifts after load, so test it on the sites you share most.

• Copy Link to Highlight: Select text, right-click, choose “Copy Link to Highlight,” then paste the URL. The link should scroll to, and highlight, the selected text on the target machine.
• PDF commenting in the built-in viewer: Open a PDF in Firefox, then use the annotation tools to add comments directly. This helps when you just need to mark up a contract fast and do not want a separate app.
• Google Lens image search (when Google is default): Right-click an image and run a Lens search. If you care about what gets sent to Google, think twice before you build this into a work routine.

Developer features: useful, but only if you can reproduce them

CSS anchor positioning is the headline.

I’ve seen frontend teams re-implement “tethered popovers” three different ways, then fight window resizing bugs for months. Anchor positioning promises a cleaner approach, but you still need to test cross-browser behavior and fallback CSS. Nightly makes it easy to experiment, it does not make it safe to ship.

• CSS Anchor Positioning: Use position-anchor and anchor() to tether an element’s position and size to another element. Test resizing the window and zooming to catch jumpy layouts.
• Storage Access Headers: Servers can opt in to un-partitioned cookies via HTTP headers. Confirm the exact header names in Mozilla docs before you ship anything, because a typo here gives you a false sense of “it works.”
• Trusted Types API: Helps prevent XSS by controlling how code creates and assigns HTML. If your app uses lots of innerHTML, expect some refactors if you want to adopt it properly.

Multi-desktop window control: the preference you’ll actually touch

This one feels niche until it isn’t.

If you live in virtual desktops, Firefox’s window behavior can drive you nuts. Nightly adds better control via preferences, including widget.prefer_windows_on_current_virtual_desktop. I can’t predict your workflow here, but I can tell you what “good” looks like: new windows appear where your cursor and attention already are, not on some other desktop you forgot existed.

• Preference to know: widget.prefer_windows_on_current_virtual_desktop
• What to validate: Open a link in a new window from Desktop 2. Confirm the window opens on Desktop 2, not Desktop 1.

Breaking changes and “don’t email me” compatibility notes

Old platforms fall off.

Nightly drops support faster than stable channels, and that’s fine, but it can surprise you if you maintain a weird kiosk machine in a corner. The article’s list calls out 32-bit Linux and older Windows versions. Verify your fleet before you roll anything out.

• 32-bit Linux: Nightly no longer supports it. Move to 64-bit if you want updates.
• Windows 8.1 and below: Nightly does not support it anymore. Treat that as a hard stop, not a suggestion.
• Local network access: If your app depends on talking to localhost or LAN services, expect permission prompts or failures until you adapt.

If your internal web app needs to call 127.0.0.1 and you cannot test it in staging, you should not run Nightly on production machines.

How I’d upgrade (and how I’d bail out)

Backup first.

Nightly breaks in boring ways: a session restore loop, one extension that refuses to load, a GPU glitch that makes your video calls stutter. Back up your profile, install Nightly through official channels, then test only the workflows you care about: internal sites, localhost tools, PDFs, and any WebGL-heavy pages. If it goes sideways, revert to your previous install and restore the profile backup. There’s probably a better way to test this, but…

• Before install: Back up your Firefox profile and list your “must-have” extensions.
• After install: Test local network access, PDF annotation, and any GPU-heavy pages on macOS.
• If it breaks: Disable the macOS GPU process via layers.gpu-process.enabled and retest, or roll back and restore your profile.

about:config flags to know

Nightly ships features behind flags. Here are the ones relevant to 145.0a1:

# Open about:config in Firefox Nightly and search for these:

# CSS Anchor Positioning (the big developer feature)
layout.css.anchor-positioning.enabled = true

# Local network access control (the thing that breaks localhost)
network.fetch.local-network-access.enabled
# If your internal tools hit localhost/LAN and break, check this

# GPU process on macOS (disable if rendering goes weird)
layers.gpu-process.enabled = true
# Set to false if you see rendering artifacts after wake

# Virtual desktop window behavior
widget.prefer_windows_on_current_virtual_desktop = true

# Trusted Types (XSS prevention)
dom.security.trusted_types.enabled

Testing Copy Link to Highlight

This is the user-facing feature most people will actually use daily. Here’s how to verify it works:

# Copy Link to Highlight uses Text Fragments (RFC)
# The generated URL format looks like:
# https://example.com/page#:~:text=selected%20text%20here

# Test it:
# 1. Select any text on a page
# 2. Right-click → "Copy Link to Highlight"
# 3. Paste in a new tab — it should scroll to and highlight the text

# Verify the URL format in the clipboard:
# macOS:
pbpaste
# Should contain: #:~:text=your%20selected%20text

# Known limitation: won't work on heavy SPA pages where
# the text isn't in the initial HTML (client-side rendered content)
# Also fails on pages that block text fragment navigation

CSS anchor positioning: real test

<!-- Quick test: save as test.html, open in Firefox Nightly -->
<style>
  .btn {
    anchor-name: --menu-trigger;
    padding: 8px 16px;
    margin: 80px;
  }
  .dropdown {
    position: fixed;
    position-anchor: --menu-trigger;
    top: anchor(--menu-trigger, bottom);
    left: anchor(--menu-trigger, start);
    background: #1e293b;
    color: white;
    padding: 12px;
    border-radius: 4px;
    margin-top: 4px;
  }
</style>
<button class="btn">Menu</button>
<div class="dropdown">Anchored dropdown!</div>

<!-- If the dropdown appears directly below the button,
     anchor positioning is working. Resize the window to
     check if it tracks correctly. -->

The full spec for text fragments is at the WICG Text Fragment spec. For CSS anchor positioning details, see MDN’s anchor positioning guide. Track all Firefox releases and support windows on the Firefox releases hub.

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