Have you tried closing the tab, waiting and reopening it?

Yep, first thing I tried. The only working solution so far is to close and restart Firefox, then reopen Gmail.

If you're experiencing issues with Gmail freezing or locking up after a few days of usage, there are a few troubleshooting steps you can try:

Clear your browser cache and cookies - sometimes, cached data can cause issues with Gmail. Clearing your browser's cache and cookies can help resolve the problem.

Disable browser extensions - browser extensions can sometimes interfere with Gmail's functionality. Try disabling all extensions and see if the issue persists.

Update your browser - make sure you're using the latest version of Firefox. Updating your browser can help resolve compatibility issues that may be causing Gmail to freeze.

Use Gmail in a private browsing window - try using Gmail in a private browsing window to see if the issue persists. Private browsing windows don't save browsing history, cookies, or temporary files, so they can help isolate the issue.

Disable hardware acceleration - hardware acceleration can sometimes cause issues with browser performance. Try disabling hardware acceleration in Firefox's settings and see if the issue persists.

If none of these steps work, you may want to try using Gmail in a different browser or contacting Gmail support for further assistance.