Try running Firefox in Diagnose Firefox issues using Troubleshoot Mode. If it functions properly in that configuration, then one of your add-ons is the culprit.

What brand laptop?
Do you know which brand scroll pad drivers are used by that laptop?

The fix is different for different brands.

@Xircal : I try in safemode not work :(

@the-edmeister : It's a ACER TRAVELMATE 5730 And I have instal the driver : Alps Pointing-device

thanks for the fast answers

Please go to Tools | Add-ons | Extensions and disable AVG Safe Search. Then restart Firefox and see if it makes any difference.

@Xircal Ow haha I see i post the Troubleshooting Information from my 3.6... sorry But I have disable the AVG Safe Search and I posted the good information :)

If you have Adobe Acrobat installed, go to Tools | Add-ons | Plugins and disable it. See post by iuliand @ http://support.mozilla.com/en-US/questions/787447

@Xircal Nope doesn't work for me ... Maybe an other problem

There appears to be a bug filed for Acer machines with the ALPS drivers. See https://bugzilla.mozilla.org/show_bug.cgi?id=605357

Thanks,

So there is a problem with the driver?

There would appear to be some compatibility issues there, yes.