Hey, long-time lurker, first-time poster here!
Just a quick FYI – I'm on macOS and kept getting the "HTTP Error in FFF 'Page not found or expired in Browser Cache (see FFF setting browser_cache_age_limit)'(428)" error when trying to use the browser cache workaround, despite setting the limit to -1.
While trying to problem-solve, I saw some mentions of macOS possibly blocking calibre from opening browser tabs in background jobs. I'm not 100% sure if that's what the issue was, but I managed to get the openers_by_scheme tweak to work that's been mentioned here before, and that seems to have fixed it for me. I just had to pass it the full path to the firefox executable file.
Code:
openers_by_scheme = {'http*': '/Applications/Firefox.app/Contents/MacOS/firefox %u'}
Since then, I've had no more error messages with loading any fics, and while it's of course still considerably slower than it was before the whole cloudflare situation, it now opens all the browser tabs it needs without any issues and is a lot faster and more reliable than it was without using the cache.
Just in case this ends up helping someone else!