[rc/rofi-pass] work around Roundcube bug in Firefox

Roundcube appears to be doing some JS magic that reacts badly with
rofi-pass: selecting an entry in rofi-pass by pressing enter causes
Roundcube to move focus from the username field to the password field,
which completely messes up rofi-pass' autotype. Strangely enough, this
doesn't happen in Chromium and I've only seen it in Firefox.