This userstyle highlights selected input-fields and textareas like Safari does.

Update (27th Jun 2009):
-fixed the problem with invisible/cut off text
(If there still remain pages were it is unfixed, tell me)

Update (28th May 2009):
-works now with 3.5b4 and above as well
-renamed the style, because Firefox 3.1 => Firefox 3.5

Note:
This userstyle does not work in Firefox 3.0.x!
A version that works in Firefox 3.0.x can be found here:
Highlight Input&Textarea (Safari Style) [Fx3.0.x]