Does Kaspersky work with FireFox\Thunderbird? Any known problems at all?

KAV works fine for me almost 2 months.

Most antivirus programs now works by monitering network connection and catching infected messages when they are being recieived or sent out. Thus, from Thunderbird's point of view, having AV and not having AV are the same thing.

If you already have virus in your mail inbox, the AV (no matter what brand) will quarantinee the inbox file so you can't access it (until you un-quarantinee it). (This will happen to all mail programs)

kaspersky does not have the support firefox
kaspersky does have the support thunderbird(pop3)

Sure?
It's no matter for KAV without support Firefox.
But KAV scan POP3 for E-mail, even Tredn PCC or Symantec NAV does the same way.
If you mean KAV without a single option for TB, it's no matter. Just choice POP3 mail and SMTP mail, it should be work fine for any mail application.

I'm using F-Secure Anti-Vius 2005.
It will block the mail if the mail included virus.

No, what you mention is called proxy scanning, it cannot scan the email attachment in advance before launch it. Antivirus software has ability to scan each email when retrieval in supported email clients.

Pleaes check : http://kb.mozillazine.org/Antivirus_software