Trying to install mozilla firefox ended up in a failed attempt because my system is "out of memory".

Further, how do I remove irreco-core-dev? I tried the command line method to remove it but with no luck.

You're trying to install apps from Devel, from the screenshots irreco-core is in devel which are not fully optified hence filling up rootfs (if you do df -h in Terminal you'll see your rootfs is full)
Solution: get rid of some unoptified apps (those you installed from extras-testing + devel)

If you've just installed a load of apps, then rebooting should free up some space on the rootfs. There's currently a bug where the install files aren't properly deleted after installing.

If rootfs is too full, will the n900 boot up?
Think the best bet might be to clear off some of these before rebooting.

And not to enable extras-devel unless you're prepared to reflash in case you mess anything up.

This is very annoying... Now even my calls don't register anymore. gpodder quit on me... can't play any sort of musics, even from my external sd card. Restarted the phone 5+ times but nothing... Tried deleting some programs but nothing... re-flash? I am going to lose data by re-flashing?

Anyone that tests applications from the extras and Dev repositories is warned over and over again that they may need to reflash their phone due to bugs. The best way is to backup your phone whenyou are happy with the configuration before you start playing again with test programmes.

What's the easiest and quickest way to re-flash the phone?

how much memory did you used up? over 2gbs? cuz that's alot and firefox isn't really big.

i tired installing openarena and it said the same error but i check my app memory and had 1.5gb left for app memory. but then i realized i had my phone plugin and it was restricting me to install it. So i unplug it and it worked fine.

Is there a way to re-flash on the N900 without another computer?

Dunno....but meaby you can try "apt-get clean" first (as root)
This will clean the packages cache?! At least it does on debian...

If it works you should have freed some space on /var
After that it should be possible to remove other packages!

But as I just send back my N900 I cannot test it for you...