One the whole I don't have many problems with Firefox, but the bad choice of mapping CTRL-U to view source is one of them. CTRL-U should be, as all good people know, mapped to delete all characters in the field back to the beginning. This is normally so handy. So I googled a bit and hey presto, a solution :
http://lists.freebsd.org/pipermail/freebsd-gnome/2004-October/008614.html
Basically the idea is to remap the key yourself. Edit the file :
/usr/X11R6/lib/firefox/lib/firefox-0.10.1/res/builtin/platformHTMLBindings.xml
or where ever it happens to be on your machine, and add the following line to the input section:
<handler event="keypress" key="u"modifiers="control" command="cmd_deleteToBeginningOfLine"/>
The best thing is that CTRL-U in the page still brings up the source.