well... I found the problem... Seems 4.0b2 doesn't like TabMixPlus...

same here . Firefox 3.6.8, Adblock Plus plugin , nothing more. Cant press enter /arrow in url. Only thing working is to open a new window.

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).
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

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.
You can use "Disable all add-ons" on the Safe mode start window to disable all extensions.
You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

I discovered two problems.. Everything was working fine last night, and then today it seems that I can't press enter in the address bar.. Problem happened after an update with AVG.. I disabled Safe Search and it worked again, but I wanted to see if I could do something to get it working as previously I had it working fine with the add on enabled but last night was forced to do a reformat and now I can't remember how I got it working.

After all that, my link scanner decided it didn't want to work in Firefox but works fine in IE, so I uninstalled AVG and have just the link scanner and security toolbar and now Safe Search works, and I can press enter in the search bar..

Right now I'm downloading AVG Internet Security 2012 (I was on 2011 before)

I found a fix without having to disable your Add Ons (assuming that it's AVG causing the problem) Uninstall Firefox and AVG.. Reinstall Firefox then install AVG.. Seems to pick up the add on better if Firefox is installed first..

Worked for me anyway