hello clverba, multiple other affected users have reported that this was an issue caused by malware on their pc. please run a full scan of your system with the [http://www.malwarebytes.org/products/malwarebytes_free free version of malwarebytes] and [http://www.bleepingcomputer.com/download/adwcleaner/ adwcleaner] (both these security tools are free). [[Troubleshoot Firefox issues caused by malware]]

Thanks Philipp! Worked like a charm.