If you want to enable spell check on every field in Firefox you just have to do the following.

Open Firefox
In the address bar type “about:config”
In the filter box type “layout.spellcheckDefault”
Change the value to a “2″, by default it should be a “1″
Restart Firefox

That should enable spell checking in all field’s in Firefox.  I don’t know what the overhead is, but I don’t imagine it being that much.