Two environments: work and home. Two platforms: Linux and Windows. Symptom is only on Windows, but spanned both work and home.
I do use the Firefox Sync feature, but of course two different email accounts for work versus home. I realize I am kind of stretching things; but, I use the Firefox sync thing across both Windows and Linux for both work and home.
Once I finally figured out how to get the Bookmarks button back, I tried to synchronize now to get the good/repaired installation of Firefox on Windows to propagate over to the broken versions. That did not work.
So: Firefox on Linux (both work and home) did not lose the Bookmarks button. Firefox on Windows (all machines) lost the bookmarks button. If it were a Firefox Sync thing, I would have thought that the repaired Windows installation would have propagated over to the other Windows machines.
Also, if it were a Firefox Sync thing, I don’t think it would have happened at the same time on two different accounts (work and home). Sure, I could have hit a bad keystroke and removed this button I wanted in one browser, and then Sync propagated the bad config. But that it happened at both work and home at the same time makes me think that it was a new build of Firefox that just clobbered the thing.
I am disappointed that 1) Mozilla didn’t have quality checks in place to catch this first, and 2) that Firefox Sync does not propagate the repair to the synchronized machines.
So, on every Windows machine, I now need to do:
(hamburger menu) –> More Tools –> Customize Toolbar
Then on the new screen, find the Bookmarks Menu button, and drag it to the toolbar. Repeat for all the Windows machines you have.