Try : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode if issue is not present then it is Extensions or Profile.

So you will have to disable all Extensions (a lot) and turn them on one by one restarting each time.

Create a new Profile for testing : https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

Please let us know if this solved your issue or if need further assistance.

Does this still happen if you disable Firebug?

See also:

• https://developer.mozilla.org/Tools/Migrating_from_Firebug

From the Firebug website:

• http://getfirebug.com/

The Firebug extension isn't being developed or maintained any longer.
 We invite you to use the Firefox DevTools instead, which ship with Firebug.next

cor-el said

Does this still happen if you disable Firebug? See also:

• https://developer.mozilla.org/Tools/Migrating_from_Firebug

---

From the Firebug website:

• http://getfirebug.com/

The Firebug extension isn't being developed or maintained any longer.
 We invite you to use the Firefox DevTools instead, which ship with Firebug.next

Doesn't help after Disabling Firebug and Web Developer Version 2.0.1 on FF ESR latest 52 64 bit. I can't even find any Firebug replaceent on any menus such as Web Developer (maybe it no longer functions as I expected). I will look for DevTools. Spending too much time simply finding and debugging addons. FF's philosophy is to hide a lot of clickable commands instead of having a copy of all clickable commands in an emergency menu. I dread the FF triple-horizontal-line menu ! I prefer the old FF menu.