Sounds like the User.js again...Rather than repeat myself have a look at this google search...this explains about - about:config as well... I have included the complete google search but the top one should help

Click on this link

http://www.google.com.au/search?source=ig&hl=en&rlz=&q=my+mouse+freezes+in+mozilla&oq=my+mouse+freezes+in+mozilla&aq=f&aqi=&aql=&gs_sm=e&gs_upl=2626217l2634181l0l2635133l27l20l0l4l0l0l779l5167l2.2.5.2.2.2.1l16l0#sclient=psy-ab&hl=en&source=hp&q=Error:+uncaught+exception%3A+Error%3A+Permission+denied&pbx=1&oq=Error:+uncaught+exception%3A+Error%3A+Permission+denied+for+%3Chttp%3A%2F%2Fwww.pogo.com%3E+to+get+property+Proxy.InstallTrigger&aq=f&aqi=&aql=&gs_sm=d&gs_upl=0l0l0l38574l0l0l0l0l0l0l0l0ll0l0&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=b19227b75f4a479d&biw=1680&bih=878

You can also stop error messages...On IE you do the following How to stop the Internet explorer script error message from showing up:

You can tell your Internet Explorer to ignore these errors when they occur and not show you an alert about them. Here is the procedure:

   Open Internet Explorer
   In the top menu, Click on Tools and then Internet Options
   Click on the Advanced Tab
   Find where it says “Disable Script Debugging” and tick the box beside it
   Now, Find where it says “Display a notification about every script error” and untick the box beside it

Not sure about Mozilla option for disableing scrip messages maybe someone has a simple solution? I did find these two solutions though and might help

http://www.ehow.com/how_6011323_disable-runtime-errors-firefox-browser.html

and

https://support.mozilla.org/en-US/kb/Warning%20Unresponsive%20script