You've got quite a few add-ons, so one thing you can try is disabling the add-ons one-by-one until the item goes away. See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

If you have many extensions then you can first enable half of the extensions to test which half has the problem. Continue to divide the bad half that still has the issue until you find which one is causing it. --- You also have a lot of network.http prefs that do not have the default value. Did you change them yourself do you have or had an extension that have changed them?

It is better to leave those prefs at the default. You can right click prefs on the about:config page and choose Reset to reset them.

To open the about:config page, type about:config in the location (address) bar and press the Enter key, just like you type the url of a website to open a website. If you see a warning then you can confirm that you want to access that page.