To enable Flash in Firefox for N900:

1. Type about:config in the location bar and press return.
2. Find the preference "plugin.disable".
3. Tap on the toggle switch to set the value to "false".

I tried to config as mbrubeck, but Flash still doesn't work with Firefox 4.

I was unable to get to 'about:config'. The browser busy spins for a second and gives me a blank page.

I was able to enable plugins by manually editing 'prefs.js' in my fennec profile directory.

You should be able to find it once you open xterm (assuming you start from your home) at ~/.mozilla/fennec/

The actual directory name will be different from another user, but it should basically look like gibberish followed by .default.

Once there you can use your favorite text editor to add the line: user_pref("plugin.disable", false);

Save, exit, start firefox 4.0 and check about:plugins to verify that they're enabled.

In the end it's barely worth doing and if you feel uncomfortable manually modifying the preferences file, you're probably better off with it disabled anyway. Expect a good deal of plugin crashing (though generally well handled thanks to ff4 it seems).

Since Nokia is very unlikely to ever upgrade the flash plugin on the N900, support for it and trying to use it will only continue to get more dubious. By this point it's usefulness is even questionable in microB where it work "better."