Recently, over on the Fediverse I said:

In Firefox 107 (and possibly earlier), if you convert a browser window into a tab in another window, quit Firefox, and restart, you lose the window-to-tab tab. It doesn't even make it into the session store. This is a potential 'data' loss, in that you can lose URLs that you wanted to read/etc.

(This is filed as bug #1801952.)

If you quit and restart Firefox normally, you have to have 'Open previous windows and tabs' turned on in Preferences in order to see this; I don't know how common that is (although it's something that I think of as essential for Firefox usage). If your Firefox just crashes (or is abruptly shut down when your session ends), this can (well, will) happen when you restart and Firefox tries to recover your aborted session.

This comes up for me because I default to opening URLs in new windows instead of in tabs, but periodically I open a burst of new windows that actually should be grouped together so I dock all all but one of those windows as tabs in the first window. People who default to tabs probably won't see this, since it's likely to be rare that you use a new window, never mind dock a window back into a tab.

You can still 'dock' URLs from windows into tabs, by opening a new tab and then pasting the URL from the window into the tab. This does preserve the new tab in Firefox's session history, although you only get the URL and not the history of the window it came from (if that history is important).

(Dumping a modern Firefox session store to inspect it or verify issues like this is somewhat involved. See for example this. I have my own tool for it, but it's a hacked up thing.)

Many Firefox issues are relatively easy to track down to a specific change using the excellent mozregression tool, and I normally do that. Unfortunately, mozregression normally expects you to be able to tell it if a given revision is good or bad with a single run of Firefox, and in this case it takes either two runs or manual inspection of the saved session state in whatever profile directory mozregression is using. So far I haven't been energetic enough to slog through the extra work that would be required to find the specific commit.