Set the pref layout.scrollbar.side to '2' to place the scroll bar at the right side.

See http://kb.mozillazine.org/layout.scrollbar.side (default is 0: user interface)

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.

Thanks for the answer.

But there is only the preference layout.SpellcheckDefault.

Funny thing is on the about:config page the scrollbar is on the right side, although I still can´t scroll with the mouse wheel.

You can check for problems with the preferences.
See http://kb.mozillazine.org/Preferences_not_saved

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

• Don't make any changes on the Safe mode start window.

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

Thanks again but still no luck.

I checked the preferences_not_saved link. Renamed the prefs.js file to prefs.jsOLD.A and restartet Firefox so it would generate a new prefs.js file. It did. But then I deleted the prefs.jsOLD.A file and now there isn´t any prefs.js file anymore.

As for the themes, there is only on theme, Firefox 3.6.12 I think. And I can´t klick any button, which means it´s already in use.

Also tried to disable one add-on at a time and restarting Firefox after that. Still no changes.

And no layout.scrollbar.side preference.

So thanks again for your help but before I make things worse, I´ll better leave it be :-))

Did you find the correct location of the Firefox Profile Folder ?

Help > Troubleshooting Information > Profile Directory > Open Containing folder

I have the exact same problem as Yerom, after an automatic update to version 3.6.14. Scroll bar is suddenly on the left side (on the right side in about:config though), and scrolling with the mouse wheel isn't working.

This is on a university computer/system, and strangely no one else using the system has this problem.

Did you try Safe mode ?

I have the same problem with FF >= 3.5 on my old computer which runs Windows XP. If you are using Windows XP or older versions, you should use FF 3.0.19