The action categories (i.e., File Edit View Histiory Bookmarks Tools Window and Help) seem to disapper at time (actuallity the font color changes to white)
This happens on multiple websites -- these categories appear to disappear, but using the mouse, you can see that they are still there, but unreadable (as are the options ion the drop down menus)
Opaite Mbohovái (1)
Hi, hold down the Option key when you start Firefox to start in Safe Mode. More - Diagnose Firefox issues using Troubleshoot Mode.
If it works in Safe Mode, disable any theme or extensions you have - 3-bar menu > Add-ons > Themes/Extensions, then restart Firefox; now if it's still OK, re-enable them one by one until you find the culprit. You may need to restart Firefox after enabling some extensions.
If it isn't an add-on, try turning off hardware acceleration - Upgrade your graphics drivers to use hardware acceleration and WebGL.
Note: If you use userChrome.css - you don't have it if you don't know what it is, this is also disabled in Safe Mode, so should be regarded as an additional suspect, and can be disabled to test by placing /* before, and */ after the file in the chrome folder, then restarting Firefox.
If your question is resolved by this or another answer, please help other users by marking the best reply as Solved. Thank you!