Cant download zip and dmg files via my browsers.

I'm running OS 10.5.8 on a 2.4Ghz Macbook pro.
Ive tried downloading several different items, Zip files & dmg's ,a game update or Vuze for example, on four different browsers - Safari, Firefox, Flock and Opera - all fail. I can download music/mp3's, Mpeg's & Jpeg's and Software Update works normally. I checked out the error consoles and got the following messages (which I don't understand) -

Error: Warning: unrecognized command line flag -foreground
Source File: file:///Applications/Firefox.app/Contents/MacOS/components/nsBrowserContentHandler.js
Line: 713

That when firefox failed to download its own update, and in Safari, anything that failed was noted in the Activity window as "plugin failed" written in red!
I'd be gratefull for some help. This is really frustrating.
Thanx
Paul.

I would run a permissions repaire via disk utility first

I would also run the 10.5.8 combo updater, if you can manage to download its dmg file after the permissions repair