User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.71 Safari/537.36 Steps to reproduce: Created a custom keymap using the Ukelele application to add the # key for simpler use on UK keyboard. Installed keymap using application and selected keymap from Mac Keyboard settings. Actual results: Works perfectly with all applications except Firefox (and other XUL apps) where cmd key operations such as Cmd-F fail to work. Expected results: Cmd-F should work as expected regardless of the keymap installed. Note that this bug has also been discussed (five years ago!) on the Ukelele forum: https://groups.google.com/forum/#!searchin/ukelele-users/firefox/ukelele-users/XJdF-EoYtIc/lpIT3dwGgxMJ
This has been partially fixed in bug 429824. The remaining work is covered by bug 1333781.