Firefox 5.0 isn't due to be released until June 21, so you are probably on the Beta Channel for updates and were offered a 5.0beta upgrade.

That Kaspersky add-on probably needs to be updated by Kaspersky for Firefox 5.0. Contact them and inquire about their schedule for updating their Firefox add-ons for this new version.

Thankyou for your answer, very helpful, I shall look into it, Jon

You're welcome.

Please click the Solved It button next to the answer that solved your Firefox support issue, it appears when you are logged in, so this thread gets marked as Solved to help other users who may have this same problem.

But the Kaspersky plugins don't work in Firefox 5. What's going on. The same problem occured between FF3 and 4.

@maclingo

Logic should dictate that if Kaspersky was slow at updating their stuff for Firefox 4.0 that the same might hold true for Firefox 5.0, wouldn't it?

I was told by Kaspersky support staff yesterday (6-22-11) that they won't fix this problem for KIS 2011 (for FF5) because KIS 2012 will be officially released in August. That's 2 months away -- longer if you don't want the hazard of being an early adopter of KIS 2012! And what happens when FF6 (or whatever) is released & is once again not compat. with KIS? Will we have to wait for KIS 2013 in another year? All you Kaspersky users... get on the phone to Kaspersky & complain... tell 'em it's their job (& why we pay good money) to have them keep their products compat. with programs such as FF!