Recently after a Firefox update, a number of search engine options were removed and it isn't possible to add/install said search engines. I used to use Yahoo.uk as my default search engine, but I can no longer do that.
I can however choose many others as the default search engine (an option if you cant get rid of SafeSurf), but for now I use Google. If I open a new page at Google.com, it automatically opens and connects to my google account and click the Settings > Search Settings, it opens the search settings, SafeSearch isn't checked.
I can make a number of changes (just try the number of results per page) and then click Save. Now check and see if those changes have been implemented in a search. If not that could indicate that it isn't recognising 'you' as a google account holder outside of when you are making the changes. So it isn't just SafeSearch that isn't working.