Hi there, Did you try clearing the phone memory (cache) and some of the internal memory? If this doesn't help try reinstalling the app from play store. Though it's unsafe to download apps from outside the play store but you may downgrade the app version by downloading any version of your choice from http://www.apkmirror.com/apk/mozilla/firefox/firefox-35-0-1-5-release/firefox-35-0-1-3-android-apk-download/ . (I never found any malware from any apps on their site that I downloaded but it's always good to remain safe. Download at your risk). Hope I was able to solve your problem. Cheers.

yes had cleared out cache. Pending reinstalling. thanks.