Because Firefox completes compositions in the last compositionupdate and then sends compositionend, Trix wouldn't perform a render because the mutation occurs while the composition is still happening, and Trix ignores mutations until they're complete.

In Chrome (and all other browsers I've tested), this is the sequence of events and mutations when composing é by pressing alt+e, then e:

compositionupdate, data = ´
elementDidMutate, composing? true
compositionend, data = é
elementDidMutate, composing? false

The same sequence in Firefox:

compositionupdate, data = ´ 
elementDidMutate, composing? true 
compositionupdate, data = é 
elementDidMutate, composing? true 
compositionend, data = é 

Before this change:

After: