Mac Main Menu Disappears After Using Bookmarks Manager

Using a 2023 Macbook Pro with M3 Apple Silicon and MacOS Tahoe 26.5.1. When I open the Bookmarks manager in Firefox and use it for a while, the main Apple menu for the Firefox app vanishes. It's only the text menu items that vanish. The icons on the right, plus date/time remain visible. Sometimes, the menu is still visible, but not operable. Clicking on any menu item does nothing. Once the menu has vanished or locked up, it remains in that state for Firefox. If I select a different app, the menu reappears for that app and functions normally. Returning to Firefox returns the vanished or locked state for the menu. Even stranger, the menu disappeared while I was writing this post. I have found a workaround. If I select the green circle and then Full Screen, then exit Full Screen, the menu reappears and is working. Has anyone else seen this? Is there some setting on Firefox that I can change to prevent this behavior? Thanks.