Hello Bergeret.

I'm guessing any menu will disappear too quickly too, and that this isn't just about the bookmarks menu.

In any case, it's possible that you are having a problem with some Firefox add-on that is hindering your Firefox's normal behavior. Have you tried disabling all add-ons (just to check), to see if Firefox goes back to normal?

Whenever you have a problem with Firefox, whatever it is, you should make sure it's not caused by one (or more than one) of your installed add-ons, be it an extension, a theme or a plugin. To do that easily and cleanly, run Firefox in safe mode (don't forget to select Disable all add-ons when you start safe mode). If the problem disappears, you know it's from an add-on. Disable them all in normal mode, and enable them one at a time until you find the source of the problem. See this article for information about troubleshooting extensions and theme and this one for plugins.

If you need support for one of your add-ons, you'll have to contact its author.

What do you do if the problem does not disappear while running in safe mode and you select disable all add-ons?

Steve, you return to normal mode and you tell me it doesn't work.

If you are trying to say that safe mode, with add-ons disable, has the same issue, then can you please try it in a new (temporary) Firefox profile and see if the issue is still present? See this article to know how to create a new Firefox profile. Please report back the results.