Firefox 152.0.6 randomly fails to access websites or perform searches

"I'm using the VPN that comes with my AV package, and turning it off makes everything work again."

Then please report that to your AV provider to fix the bug, they should be able to replicate or support you as paying customer. If Firefox receives invalid HTTPS, it can't do anything about that. If they don't know why it's happening, they can always get in touch with the engineers directly using bugzilla or matrix, that's the beauty of open source.

If you feel like doing the work for your vendor, you can open a bugzilla ticket, and work with the networking/security engineers there to capture your encrypted traffic samples to help understand the compatibility problem with your security offering. (Though it might end up inactionable without a change on their side eventually.)

If you're tempted to do even more math, you can try a simple mozregression run to launch a set of old versions and verify whether Firefox release timeline is a factor in the resulting (in)reliability of the connections received.

I did immediately suspect the AV provider, but the difference is that this doesn't happen with the other two browsers. Something specific to Firefox is going on here.

However, I didn't know about mozregression, so thank you for pointing that out. I will certainly give it a try.

The networking stack is indeed different. Basically any other mainstream browser you might have is pretty much the same thing, just with different sprinkles on top. So if they make sure they can inject into chromium, they can pretty much inject into all the various clones.

There are however issues logged almost every month with various instances of AVG, Avast, Norton, Eset, Bitdefender, Sophos, SentinelOne breaking this protocol or stalling such and such handshake or resumption, or dropping frames for this and that cloud provider etc. — because usually, the way these platforms work, is not exactly secure or private one; so they have a hard time trying to trick Firefox into thinking their shenanigans are secure when they aren't etc. — and sometimes these clumsy attempts bottom out with a user facing error like this:/

Or, in shorter, the explanation is in Fix problems connecting to websites after updating Firefox article — they might be too slow to update their internal signatures or dynamic rules to adapt to e.g. unplanned Firefox security releases, in case they're targeting apps through an allowlist one by one. 🤷