Mozilla released an emergency update to Firefox 35 on January 26, 2015 with Firefox 35.0.1. There are several fixes including for startup crash with this release. See the release notes for full details.
Users may be prompted to update to the newest release (35.0.1) of Firefox or can do so manually within Firefox by going to Help > About Firefox and following the update prompts. Users may also manually download and install the newest Firefox update (35.0.1 via the getfirefox.com site. The next scheduled update for Firefox is February 24th, 2015 with Firefox 36.