Works for me...

I also have it set on my FreeBSD with Firefox 30-something same method with "FreeBSD".

But note that it does not appear when you post, only after you return to the page.

Try again. I've put it in about:config this time. No idea why it worked on my desktop, but not my laptop.
YES! Strange.
astrogeek
See my post #13 for about:config vs prefs.js editing.

Turns out there is a better way: put your override in user.js (in the profile directory, where prefs.js resides).

If you have a user agent override that works in prefs.js just exit Firefox and
then remove that line from prefs.js.

Right now, mine looks like
Restart Firefox and check the result with http://whatsmyuseragent.com/

Hope this helps some.

[EDIT]
Do the same for Seamonkey.

In either case, you can freely edit user.js (when the browser is not running) and most updates are just the version number (which you can get from about). Use a text editor.

You can find the default useragent by removing any edits you've done in prefs.js, restart the browser and go to http://whatsmyuseragent.com/ from which you can copy the user agent string into user.js. Be sure to add the user_pref("general.useragent.override", and other items as shown in the example above.
[/EDIT]

Took a little while to puzzle this out, because I was letting myself get carried astray by an existing "general.useragent.extra.firefox" attribute in my about:config (left over from older browser installations).

Setting that did nothing.

I had to right-click and select "New" to create a new attribute called "general.useragent.override". Setting that to the useragent string I wanted worked great.

Feel free to check yours using http://ciar.org/ttk/echo.cgi if you don't like the annoying advertisements and graphics on that other page. It will be in the "HTTP_USER_AGENT" environment variable.