This issue can be caused by an extension that isn't working properly.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

This is helpful. Thank you. Can you give a bit more detail about the step where I selectively enable the add-ons & extensions? When I go to the Get Add-ons page, where does it show the Add-ons that have been disabled? When I go to the Extensions page, it does, unlike the Add-ons page, show the specific extensions that have been disabled, but I don't see where I click to enable a specific add-on.

Thanks.

You can see which extensions are installed and enabled on the Firefox (or Tools) > Add-ons > Extensions page.

Did you look at https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes ?

I understand the concept and the steps to take, but when I go list of extensions that have been disabled via safe mode, there is no button "Enable", only buttons for "Disable" and "Remove." If I click on Disable button, it switches to Enable...but that implies that it already was in an enabled state. (And if I think click on enable, nothing happens.) How can that be? It says "all add-ons have been disabled by safe mode" at the top of the screen.