Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: "3-bar" menu button or Tools > Add-ons > Appearance
• Do NOT click the "Refresh Firefox" button on the Safe Mode start window

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

Try the Firefox version from the official Mozilla server if you currently use a version from the repositories of your Linux distribution to see if that makes a difference.

• https://www.mozilla.org/en-US/firefox/all/

I have fixed this (im using linux mint)

Create this ~/.config/gtk-3.0/settings.ini and add [Settings] gtk-primary-button-warps-slider = false

Now left mouse-click does not couse the bar to warp to mouse location but jumps one page at a time, that is the way i want.

I would like to see bar warping on right-click but that is something to look into.

Apparently that left-click warp is a new feature of GTK3. I dont know why that was changed. It's not consistent with other applications behavior.

Anyway. Problem solved.