Google in particular changes their certificates several times a week, which is really bad practice - you can't be sure if the new certs are valid or MITM attacks. This is something the existing browsers do not check for. TC domains don't offer https though?
The check itself can only be disabled at compile-time, otherwise some malware could disable it without the user knowing. You'd comment out the wk_set_ssl_func line. This will leave you less secure, but if you use it for regular browsing on an old machine (no banking, etc), it would be fine.
edit: Fifth is pretty fast to build, when you use the existing webkitfltk-dev extension. Should be no longer than a minute even on an old machine.