User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:82.0) Gecko/20100101 Firefox/82.0

Steps to reproduce:

Always reproducible. I click on any toolbar bookmark folder and then right-click a url within that folder.

Actual results:

Each option in the drop-down menu (Open, Open in a New Tab, Open in a New Window, New Bookmark etc) starts flickering as I run the cursor over them. This happens only when right-clicking on urls nestled within any particular folder in the bookmarks toolbar. It does not happen when I right-click a bookmarked url or folder directly from the toolbar.

Expected results:

Each option should remain a solid blue and only change as I move the cursor from one option to the next.

Note: the screenshot doesn't (can't) show the flickering but I've added it for clarity.

A bit more info. I've tried the following things w/out success ...
Restarted the browser with Addons disabled.
Refreshed the browser.
Tried a different mouse.
A complete uninstall/reinstall of FF with only the bookmarks.js file exported/imported.

I thought the bookmarks.js file might have been corrupted, so I went into the new Profiles folder and removed 'bookmarkbackups' and 'places.sqlite'. After restarting FF all bookmarks were gone (as expected) so I created a new toolbar folder, added 3 or 4 urls and tried once more, but again the same flickering problem.

Screen recording of the problem.

https://tinyurl.com/y2zellee

Able to reproduce on macOS 11.0.1 Beta (20B5012d).

Re bug 1261327

Workaround:
Remove the download toolbar button from toolbar.

It was in the toolbar so I removed it, but no change unfortunately.