Bugzilla

I can reproduce consistently on both Linux and Windows with these steps:

  1. Download a release Firefox and create a fresh profile with it. Open it and:
    - Pin a top site as a breadcrumb (to confirm the correct profile loads later)                                                                                                            
    - Verify browser.newtabpage.trainhopAddon.version is set in about:config
  2. Restart Firefox on that same profile — this is critical. The restart promotes the trainhop XPI from extensions/staged/ to extensions/ and writes the XPI's absolute path into addonStartup.json.lz4.
  3. Close Firefox.
  4. Copy the profile to a new location. Keep the original directory intact.
  5. Run the same downloaded release Firefox against the copied profile — do NOT use a local build here, as it will silently fix the stale path on first run: `/path/to/downloaded/firefox --profile /path/to/copied-profile/1, these paths need to be absolute, in my experience.
  6. Check: newtab should be blank on Windows. If not check that pinned top site confirms the right profile loaded, otherwise something went wrong. If you see the pinned topsite, you loaded the correct profile, but otherwise didn't trigger the bug.

I can reproduce consistently on both Linux and Windows with these steps:

Build an xpi
1. In browser/extensions/newtab/manifest.json, bump the version by one minor (e.g. 150.2.0 → 150.3.0)
2. Build and run firefox
3. Find and copy obj-x86_64-pc-linux-gnu/dist/xpi-stage/newtab@mozilla.org.xpi (obj-x86_64-pc-linux-gnu might be different for you if you are on a mac), save that somewhere.
4. Reset browser/extensions/newtab/manifest.json

Install the xpi
1. Build and run with a fresh profile
2. In about:config, set:
   a. "xpinstall.signatures.required" to false
   b. "browser.newtabpage.trainhopAddon.version" to whatever you set it to in step 2 (for me 150.3.0)
3. In about:addons, install the xpi we stashed from before.
4. Restart, about:support should list the installed xpi version (for me 150.3.0)

Trigger the bug
1. Go to about:support
2. Open Profile Directory
3. Copy the profile folder
4. Paste it somewhere, (I used mozilla-central)   
5. Build and run with the new profile dir (for me this was ./mach build && ./mach run --profile /home/scott/mozilla-unified/14bshfzh.blank/ --jsdebugger)