Keystroke Alternative Rated 5 out of 5 stars

Alternative on your Mac while you are waiting for an update:

Firefox menu>Services>Services Preferences...
Select 'Application Shortcuts' press the '+' to add a new one.
Enter 'Look Up in Dictionary' (get the capitalization correct), add your short cut '⌘;' (or the default that works in TextEdit and Safari '^Shift-D').
To use, select a word in Firefox, the press '⌘;' and the Dictionary.app will open to that definition.