Can you try another mouse in case there is a problem with this mouse and the left mouse button might be stuck in pressed state or too sensitive?

This is not a feature in Firefox.

Could be the mouse or the mouse driver settings.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

I tried safe mode, and that didn't help. I tried resetting Firefox, and that didn't fix it. I looked in the Mouse settings on the computer, and didn't see an option to change that. It does look like it's doing it other places than Firefox, so I will look again. The change was very abrupt, this morning, and I didn't do any updates or anything like that.

Thanks!

Did you try another mouse?

Did you try to reboot the computer?