Terry - This is also affecting Winders and Mac users, using several different browsers. Also friends here in Sydney using XP-Pro SP3. Annoys my teenage godson, who for some reason likes pix of ladies who can't afford adequate clothing....
Cross-verified by my XP-Pro install in V-Box in PCLOS - it's doing the same thing.
As you might recall, until going to this new PC a few weeks back, and the current PCLOS and KDE4.x (to get the current Kdenlive, RawTherapee, so on) - I was using the previous PCLOS with KDE-3.5 and a much older version of FireFox - which combo had no problems whatever with turning Google Safe-Search OFF.
I doubt that it has anything to do with "current" PCLOS or the "so-charming" KDE4.x - it's cross-platform, and seems to do the same "auto default to Moderate Safe-Search" with several browsers.
It has to be something Google is doing - it doesn't happen with Yahoo or AltaVista.
As Linux has more and better Options, Choices, Settings and Functions, than the other systems - I was hoping that some of the very cluey OpenSource folk would have discovered or invented a workaround for it...
For now, I can use Yahoo Search - as that is holding its Search Direct OFF, Safe Search OFF, and Search Scan ("dangerous sites warning") OFF, settings.
As an example of what's happening - I tried to Google latest "news" about a certain country in the news recently - and indeed Google did show a lot of results - but none from that country's "official" news (propaganda) network. The same Search in Yahoo had PressTV come up as the 5th return. I do like to look at what "all sides" are saying re events - and if PressTV's "news" is a little like a modern version of the "Goon Show" - I don't mind having a bit of a laugh, either.... At least I know what they're saying....

Regards, Dave.