I've noticed that since Mozilla v1.21 the fullscreen option in KDE3.x no longer seems to be working ass fullscreen but continues to be bound by it's window that does not change it's size ... as a result any other visible apps still remain visible.

Anyone noticed that too?
Any solutions to that?

And it's not jost KDE ... same occurs with window managers such as: TWM, and BlackBox.

Is this problem uncommon or nobody uses the Mozilla's fullscreen option in Linux?

fullscreen works fine with me (although I'm using Mozilla 1.5a and i recommend you upgrade yourself :P) although i never use it, i dont see a point /shrugs

Fullscreen works fine for me, I really enjoy surfing the web in FS mode as sites look quite a bit better on my flat-panel that way; whether or not I get the entire screen depends on the Window Manager that I'm using however. On IceWM it takes up the full screen and works fine, under GNOME or KDE the taskbar or something else takes up some of the screen real-estate. This has been the case with all recent versions of Mozilla as far as I can recall, I'm currently using Moz 1.4, (haven't been downloading the latest releases as quickly anymore as 1.4 is amazingly good).

It seems that the fullscreen option is only supported by some WM / DM ... I just tried Mozilla 1.5 rc2 and a, and both behaved in the same way under KDE and BlackBox as Mozilla 1.4 (where mozilla did not takeup the entire screen).

I'm not sure if I wont to switch to IceWM to get that feture working.

Perhaps if I was to call mozilla without decorating it with WM/DM window then that would be a workaround for fullscreen ... I'll look into some documentation on tha now.