To disable this back action in firefox do the following:
about:configinto the url line.
Accept the warning.
Enter mousewheel.withshiftkey.action into the filter at the top of the page. You should get one line with the same name and the value set to 2.
Right click the line and change the value to 0.
Other values you can set for this field is:
* 0 - Scroll document by a number of lines (given by the numlines property)
* 1 - Scroll document by one page
* 2 - Move back/forward in history
* 3 - Make text larger/smaller
* 4 - Scroll document by a number of pixels (given by the numlines property)
Thanks to http://support.mozilla.com/bn-BD/questions/701079 and http://kb.mozillazine.org/About:config_entries#Mousewheel. for those.