hi, a third-party software tool on your system apparently has overwritten the useragent of your firefox (that's the bit of information a browser identifies itself to websites). please see here in order to reset the user-agent settings in firefox and fix this problem: How to reset the default user agent on Firefox

Thanks for your reply "philipp", but it didn`t work. The only "useragent"- entry offering a reset, was "general.useragent.override". After I did reset it, the very same entry was shown, when I opened it up the next time:

"Mozilla/5.0 (Windows; U; Windows NT 6.2; de; rv:1.9.2.6) Gecko/20100625 Firefox/ Anonymisiert durch AlMiSoft Browser-Anonymisierer 49691822"

Looked up my software - but can`t find neither any "AIMiSoft Browser-Anonymizer" nor "Gecko" on my PC.

in this case please try to generally refresh firefox and see if this can address the issue...

Sorry but I´m reluctant to refresh, since I`d loose all my addons which are working and in case refreshing doesn`t solve the problem I`d have no opportunity to install them again.

do you happen to have a file called user.js in your profile folder? if yes, try to delete it... Profiles - Where Firefox stores your bookmarks, passwords and other user data

I deleted user.js in my profile folder - no change.

afterwards you'd have to reset the general.useragent.override pref one final time - hopefully it will stick then

Yippee! It worked! And thank you so much for your patience with me, philipp! Best greetings from Austria, Alf Bear