User Agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:34.0) Gecko/20100101 Firefox/34.0 Build ID: 20141126041045 Steps to reproduce: Linux: in a fresh environment, create the directory '~/.cache/' to be empty but accidentally unwritable; then attempt to start firefox. Actual results: Dialog box: "Your Firefox profile cannot be loaded. It may be missing or inaccessible." And no way to start firefox. Expected results: I would have liked, ideally, an error message that points to the problem with '~/.cache'; instead I got a message about the profile, which is not the problem (and of course it took some time until I figured it out).
Armin, although the error message might be too generic for this specific case it describes exactly what is happening: Firefox doesn't have write access to your profile, hence the error message and the normal inability for Firefox to start. However, I partially agree with you that maybe it would have been nice to have a more pointed error message for this specific case.
I would argue that my profile is in ~/.mozilla/firefox, not in ~/.cache/mozilla/firefox.