Check your zoom controls.

<Control> (Mac=<Command>) + (plus) enlarge - (minus) reduce 0 (zero) restore

Hover your mouse over the menu and see if the mouse wheel does anything. Also, try using the arrow keys.

Thanks, but those suggestions don't address the issue.

I'm imagining that Firefox could draw the bookmark menu correctly, with a completely visible highlighted choice, while taking other settings into account.

My goal is to use the alpha keyboard to find a bookmark, without switching to arrow keys or the mouse.

Yes, I see what you mean. It's almost like the menu scroll position doesn't account for the bottom scroll button. You need to arrow down to see it in full.

I found a bug in the tracking system (#1671000) which sounds a lot like this issue. It was reported in Firefox 81 and has been fixed in the upcoming Firefox 95, due out Dec. 7th.

Not sure there are any workarounds to use between now and then, unfortunately.

Thank you for acknowledging the problem. (I reported it once before on 9 October 2020, so maybe you that's the bug you found.)

It's good to hear that it will be fixed!

You probably need to disable the code in userChrome.css that hides the scroll buttons if you are currently doing this. If you also use code with a negative margin-bottom then leave this property at '0px' and only adjust the margin-top in case having both set to zero isn't sufficient.

Thanks for the reply cor-el.

I'm not an under-the-hood user, and I don't recognize what you're talking about, so I'm guessing that I haven't made any of the changes that you suggest undoing.

If you can reproduce my problem by *making* those changes, then maybe you're on to something. ;)

This bug occurs without any customization needed, just a menu long enough to require a bottom scroll button.

Belated thanks to jscher2000 for the corroboration last month.

Meanwhile, my Firefox upgraded to "91.4.0esr (64-bit)" last week, but the bug persists. Does that make sense to you?

The Extended Support Release series doesn't get all of the same fixes as the regular releases, only the most critical ones. You could file a new bug requesting the fix from bug #1671000 be applied to the ESR series. I don't know whether they will.

https://bugzilla.mozilla.org/

Thank you jscher2000! I'm not sure how I got into the ESR channel, but switching back to the regular channel did fix the bookmark menu issue for me. Hopefully no older non-ESR problem will recur. I really appreciate your help. :)