User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:42.0) Gecko/20100101 Firefox/42.0 Build ID: 20151029151421 Steps to reproduce: Place cursor over the word on any webpage and press Command-Control-D. Actual results: Nothing displayed. Expected results: Popover with a dictionary article should have been displayed. Said shortcut is system-wide and should work in any Cocoa application, which Firefox is for a few years already.

Loic, two things: I'm retired, and no longer work for Mozilla. If you're in doubt who to CC on a Mac bug, it's probably best to CC Markus Stange, who's now the OS X Widgets module owner. I'm quite sure this bug is a dup of another bug specifically concerning OS X El Capitan (10.11). Please look for it.

Steven, As for duplicate, I looked for it before creating this one but didn't find it. And you are right, it seems to be specific to El Capitan, I've just checked it on Yosemite and confirm that it is absent on 10.10.

Here is Console log: 11/10/15 10:00:13.493 AM LookupViewService[493]: <NSXPCConnection: 0x7fb1d3e1a5e0> connection from pid 491: Warning: Exception caught during decoding of received message, dropping incoming message. Exception: Exception while decoding argument 0 (#2 of invocation): <NSInvocation: 0x7fb1d3f1a3b0> return value: {v} void target: {@} 0x0 selector: {:} bootstrap:withReply: argument 2: {{?=@@@@{CGRect={CGPoint=dd}{CGSize=dd}}Icc@@@}} <00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000> argument 3: {@?} 0x0 (block) Exception: decodeObjectForKey: class "TitlebarAndBackgroundColor" not loaded or does not exist 11/10/15 10:00:13.493 AM firefox[491]: Lookup: Unhandled exception 'NSObjectNotAvailableException' caught in +[LULookupDefinitionModule _showDefinitionForTerm:atLocation:options:]]