How can I anonymize my browser useragent, yet still be counted as a Firefox/Ubuntu user?

Partially anonymizing your user agent actually makes your header more unique, not less. (Think of a stripe-less zebra in a herd.)

However, if you insist I would start by removing all specific minor version numbers as they can be used to target known vulnerabilities. I would leave alone the strings likely used by applications to determine browser compatibility.

So

Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.7) Gecko/20100106 Ubuntu/9.10 (karmic) Firefox/3.5.7

becomes something like this:

Mozilla/5 (X11; U; Linux i686; en-US) Gecko/2010 Ubuntu/9.10 (karmic) Firefox/3.5

You can use this addon to change/create custom User Agent's.

Since it's Ubuntu, I would imagine there are many people with the same Firefox version. So your original is actually better than, for example, Mozilla/5.0 (Linux) Gecko Ubuntu Firefox, because the original is a lot more common.

First, why would you want this? Nobody can tell who you are from your User-Agent, your IP address is far more interesting.

You could change your version numbers, but that won't achieve much.

To get to general.useragent.override, you type about:config in your address bar, give your consent, and then search for it, right click and edit.