debian apt – Debian 8 apt-get Update Showing 404 Not Found

About your requirements, Iceweasel is the supported Firefox derivative (fork), I'm currently running debian as my desktop OS at work and use iceweasel every day, no problem. Gnome3, I think it'll be available on the next stable release, BTW what release are you running? Squeeze? If so I think (IIRC) Wheezy will have it. And finally, about graphics performance/quality, that depends a lot on your graphics card and its driver, but if you think of it like having transparencies, windows closing with fancy effects and so on, you'll need a moderm desktop or compiz (work with gnome2) which I think its available on stable (wheeze).

I have modified a bit your sources.list for wheeze, do you mind to test it and report back?

deb http://http.us.debian.org/debian/ squeeze contrib non-free main
deb-src http://http.us.debian.org/debian/ squeeze main

deb http://security.debian.org/ squeeze/updates main non-free contrib
deb-src http://http.us.debian.org/debian/ squeeze/updates main

# squeeze-updates, previously known as 'volatile'
deb http://mirror.cse.iitk.ac.in/debian/ squeeze-updates main non-free contrib
deb-src http://mirror.cse.iitk.ac.in/debian/ squeeze-updates main non-free contrib

# 3rd party repositories
deb http://packages.dotdeb.org squeeze all
deb-src http://packages.dotdeb.org squeeze all
deb http://www.deb-multimedia.org squeeze main non-free

If you happen to be using wheezy or sid you'll have to change all squeeze ocurrences for the one you're using.

Please, backup your sources.list before replacing it, then test it as follows:

Refresh caches

# apt-get update

Search package

# apt-cache vlc

Install package

# apt-get install <package_name>

If find trouble, please report back with output for those three commands, or at least for the first and last.

Also, if you are already using Wheezy or sid, forget what I said about Gnome3 not being available, it should be there but wheezy is yet to be release and sid is always the development branch.

I don't understand why you are doing here. Why do you have a preferences setting for stable at all if you are running a stable system? As far I know, no preferences setting is necessary for stable in that case.

You don't explicitly say whether you are running a stable system (you really should say so), but if you are not, then I really have no idea what you are doing.

And if the release is on stable, then the usual thing to do for testing and unstable is to set their preferences to less than 100. I usually use 50.

And if you want to downgrade to stable, just do the following (assuming sane settings like the ones above) to downgrade pkgname1 and pkgname2:

apt-get install pkgname1/stable pkgname2/stable

This sets the specified packages to the target release stable.

Incidentally, mixing testing and/or unstable packages with an unstable system is generally a bad idea unless you know what you are doing. Some of the time it is Ok, but most of the time you need to use backports, either from Debian, or self-made.