That is super weird. Do you happen to have any custom themes installed? You can find them under add-ons.

Next, do you make use of userChrome.css or the Stylish extension?

Any of these could make the menu invisible. Try switching to the default theme or disabling these extensions if you have them.

Otherwise, does the error still occur in private mode?

Does this only happen if you visit some specific websites ?

I noticed that some changes have been made that seem to affect the z-index of some components and that might be causing the list to be no longer visible.

No custom themes installed - currently using the default. My only extensions are 1Password, Amazon Assistant, Grammarly, and Facebook Container. I'll try disabling Amazon & Grammarly and see if that makes a difference.

You can just restart Firefox in safe mode and see if the issue persists. This will disable extensions and some setting for the duration of safe mode.

☰Menu → Help → Restart with Add-ons Disabled

So it appears that YouTube (or the specific channel analytic page) is the culprit. Not sure how to fix it, but at least I've identified the source.

alifreeman93 said

So it appears that YouTube (or the specific channel analytic page) is the culprit. Not sure how to fix it, but at least I've identified the source.

Are you referring to https://www.youtube.com/analytics

I can't reproduce the issue. Though, the top bar has a z-index set, so I'm guessing cor-el was right about the issue.

What drop-down list is this about ? If you open the list from a toolbar button on the Bookmarks Toolbar then is is probably possible to increase the z-index of the drop-down list via code in userChrome.css.

See also this tread for a similar z-index related issue.

• /questions/1276942 Bookmark sidebar is only showing up in the console window. How to restore original view?

Try to add this code to the userChrome.css file below the default @namespace line to see if that has effect.

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#personal-bookmarks menupop {
 z-index: 2147483647 !important;
}

It is not that difficult to create userChrome.css if you have never used it.

The first step is to open the "Help -> Troubleshooting Information" page and find the button to access the profile folder.

• https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox

You can find this button under the "Application Basics" section as "Profile Folder -> Open Folder". If you click this button then you open the profile folder in the Windows File Explorer. You need to create a folder with the name chrome in this folder (name is all lowercase). In the chrome folder you need to create a plain text file with the name userChrome.css (name is case sensitive). In this userChrome.css text file you paste the text posted. On Mac you can use the TextEdit utility to create the userChrome.css file as a plain text file.

In Windows saving the file is usually the only time things get more complicated because Windows can silently add a .txt file extension and you end up with a file named userChrome.css.txt. To avoid this you need to make sure to select "All files" in the dialog to save the file in the text editor using "Save File as".

You need to close (Quit/Exit) and restart Firefox when you create or modify the userChrome.css file.

See also:

• https://www.userchrome.org/what-is-userchrome-css.html
• https://www.userchrome.org/how-create-userchrome-css.html

In Firefox 69 and later you need to set this pref to true on the about:config page to enable userChrome.css and userContent.css in the chrome folder.

• toolkit.legacyUserProfileCustomizations.stylesheets = true

See:

• https://www.userchrome.org/firefox-changes-userchrome-css.html

Thanks for the detailed instructions, cor-el. Unfortunately that did not solve the problem. Oh well. The problem is annoying but not impossible to deal with so I guess I'll just work around it.