it won't matter for firefox per se as disabling MPROTECT already did all the damage . however you'll lose a brute force prevention trigger event which may or may not matter to you.
just a quick update on the situation: it appears that FF36+ have fixed this problem (https://bugzilla.mozilla.org/show_bug.cgi?id=1091912), so you should be able to enable PAGEEXEC/SEGMEXEC again. it would also be nice if anyone could confirm that it actually works now .