A new version of KeeFox has just been approved on the official Firefox add-ons site.
This version supports KeePass 2.17 (and earlier). There are a few other improvements too - full details are in the release notes for version 0.9.3a on this page: https://addons.mozilla.org/en-US/firefox/addon/keefox/versions/
I recommend installing the updated version of KeeFox while still running version 2.16 (or earlier) of KeePass. If you have already upgraded to KeePass 2.17 you will need to “paste and go” this URL in your location bar and click “Upgrade KeeFox”:
chrome://keefox/content/install.xul?upgrade=1
A change was made to KeePass in version 2.17 which means that KeeFox 0.9.2 and earlier no longer work. Specifically, the KeePassRPC KeePass plugin no longer loads (it gives an error message saying it is incompatible with the new version of KeePass). This sort of plugin incompatibility issue was supposed to have been eliminated with the new “plgx” plugin format but there are evidently still a few edge cases that can cause a problem and KeeFox was the unlucky plugin this time around.
Dominik (the main developer of KeePass) has kindly agreed to try to give me advance notice of future KeePass releases so emergency fixes like this can be developed before the new KeePass version is made publicly available. Neither me nor Dominik can make any promises but we think the 2.17 upgrade problems are atypical of past and future KeePass upgrades.
Special thanks have to go to the addons.mozilla.org team (particularly Jorge Villalobos) for fast tracking this new version through their thorough and often lengthy review processes!
Sorry for any confusion or inconvenience this may have caused you.
NB: A few bits of text are “NOT TRANSLATED” for non-English users - they shouldn’t be crucial to understanding the add-on and will be replaced with correct translated versions as soon as possible. Sorry, this decision was the lesser of two evils!