I downloaded Firefox 1.5.0.2, and have some weird things happening since, one of the problems is when I go to google something and enter the first letter of a site that I go to regularly and I click on the option I want in the drop down box it normally fills in the rest of the address, but since I updated to the 1.5.0.2 it occasionally fails to complete the address and leaves me with a blank address box, but will complete it on a second try.

Another is when I want to highlight, copy from one site, and open another google page, right click to paste the option isn't availabe, and I have to go back to the original site and copy again.

Is it just me, or is anyone else experiencing anything like this?

Just for the record, I have the following, and have run them only to find the usual cookies.

AdAware
Spybot S&D
Ewido Suite
CCleaner
AVG
Sygate

Do you have this same problem with Internet Explorer?

If it's just Firefox causing problems, it could be a corrupt install.
I'd suggest a simple reinstall.
Delete Firefox using Add or Remove Programs.
Go to C: / Program Files, and delete the Mozilla Firefox folder.
Download a new copy of Firefox.
Now, reinstall Firefox.

Do a virus & spyware scan with the programs you listed above. Then tell me the results. This maybe caused by spyware...it might not have anything to do with Firefox at all.