Unfortunately this is a known bug for Firefox for iOS 1.2. This will be addressed in an upcoming bugfix release.

The only way to get around this bug right now is to delete and reinstall the application.

Thankyou for your response.

I tried to delete and reinstall however this did not solve the problem. I think it says that this is at version 1.4??? Also, it appears the problem only occurs when syncing to my firefox on PC. I can log in to sync if my desktop is disconnected ie between my 2 ios devices but as soon as my desktop is added the crashing starts. My desktop is windows 10