We've started looking into this, and have tracked it down to a combination of a Caching change about a week ago, and seemingly a Firefox issue with mixed-content pages which causes the form token cookie to be reset more often.
One of the sticking points is that Trac creates a session in it's DB (And browser cookies) for every visitor, even if they're not logged in.. which as you might expect, means it's not very cacheable. Thanks Trac!
We've got some config in place to allow Trac pages to be cached (ignoring the cookies) but it's also affecting logged in users.
Stay tuned, we'll get this sorted.