Restoring Firefox to its default settings can solve most of "Firefox browser" related problems.
There is two ways to reset Firefox

Method (A):

1- Open Firefox.

2- In the menu bar click Help then troubleshooting information...


 3- Reset Firefox.


 4- Reset Firefox.(Done)
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Method(B):

1- use safe mode:
there is many ways to start Firefox in safe mode, two of these ways are:
in command prompt tape this command "start firefox -safe-mode" the (Enter), or in Run dialog box tape this "firefox -safe-mode" then (OK), use these commands without quotes"".




2- Reset Firefox.

 

3- Reset Firefox, (done).