How to avoid Sync to cause crashing firefox?

I think the crash happen because of your addons, check your addons are uptodate! Remove the unwanted addons

They are all up to date. So should I disable all and enable one by one and try again?

The problem is, that i cannot start firefox anymore (but in safe mode), because it instantly crashes.

Create a new profile as a test to check if your current profile is causing the problems. See "Creating a profile": *https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles *http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues Profile Backup and Restore *http://kb.mozillazine.org/Profile_backup *https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles *http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox