It is related to Desktop mode browsing that is now enabled by default for large devices. This can be controlled via a preference under Settings in "Site settings".

Dear TyDraniu, Thanks for your prompt reply. I disabled desktop mode in site settings and it does't change this behaviour. The size remains a third too big of what to expect. Cleaning cookies, history and so on didn't help. I think this behaviour occured after installing this new FF version. Stephan