RES is not known to crash Firefox. With over a quarter million users on Firefox, it's possible you have some unique issue to your system. Blaming the extension for this in a review isn't exactly reasonable.

Have you tried asking for help in /r/RESIssues ?

Are you referring to the Reddit redesign? (Does the top left corner show an orangered circle with a snoo face?) RES is adding support for the new layout, but isn't responsible for it. You can switch back to old Reddit by using old.reddit.com or turning on "Opt out of the redesign" in https://www.reddit.com/settings