Kaspersky installs a certificate for Firefox so that all https traffic can be checked. If you click on the lock in the address bar in Firefox, and then the arrow next to “connection secure”, you will see a certificate from Kaspersky. Apparently that causes problems with some websites. That was why I quickly left Kaspersky again. And I also don’t want a man-in-the-middle check of https traffic by my virus scanner at all.
But there are more AVs that it doesn’t seem to matter if they break Firefox. For example Bitdefender causes the Firefox Widevine plugin to crash on Netflix website. And Windows defender scans the Firefox on-disc cache a lot, making Firefox noticeably slower.
At the moment I use the AV of F-Secure that I get with my Ziggo subscription, I have not seen any major problems with F-Secure. And is one of the few AVs made by a European company, I trust that more than a company from Russia or the US.