What menu are you talking about?

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

FredMcD said

What menu are you talking about? Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

I mean the menu bar! In safe mode the problem isn't there anymore.

It could be the work of one of your add-ons. Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control>(Mac=<Command>)<Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

FredMcD said

What menu are you talking about? Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Something happened after the safe mode. The next I restarted Firefox, something must have been changed because the problem is gone! Thank you for your swift response.

Glad to help. Safe Surfing.

Sometimes starting in Safe Mode overwrites a corrupt file.