Hi joerstewart, there are some bugs on file from last month that this is an issue on MacOS Catalina when Firefox is not running and it is initially started using a link in an external application instead of starting it from the dock or a Firefox shortcut. In those bugs, users reported one window opens that has the menu labels, and a second window opens that contains the linked page is missing the menu labels.

Does that fit the pattern you are seeing?

(I have no idea when this might be fixed because the bugs do not yet show that the cause of this has been tracked down.)

See also:

• Bug 1603956 - macOS Catalina: Menu labels missing when cold opening Firefox from link or other app

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)