I've seen a few posts about this, but unless I missed something, the solutions presented aren't exactly ideal. For example, one recommends changing the "general.useragent.extra.firefox" string to add "Slackware" to it. However, note that the Firefox version is also in that string, so you'll have to remember to keep that updated every time you upgrade Firefox, or else your useragent will be out of date. That's not a huge issue, of course - it doesn't really matter - but for some of us, we're a bit OCD about things like that

Therefore, here's my proposal that *seems* to work:
Open a new tab and navigate to "about:config"
Right-click --> New --> String
For the preference name, enter "general.useragent.extra.slackware" and then enter your preferred extra text to be added to the UserAgent string -- in my case, I used "Slackware Linux"

Best I can tell, the Slackware logo shows up as expected. YMMV.

Well, now it isn't working. I *swear* it was a little while ago... :/

Testing... appears not to work for now.

It worked after reloading the page.

Edit: maybe it has to do with the browser cache, I don't know.

and now it is. WTF?

EDIT: and now it's not again. This makes no sense.

Got to test this.

Seems to work for me in 100% of my posts so far. ;P

Edit 2:

Is there a bug here? (In LQ, not firefox). When I press save after modifying my post it disappears. It reappears when i refresh however.

Thanks Robby for the suggestion.

I've also had a problem with the logo appearing sometimes and not appearing other times :/.

Hmm let`s see if it works for me

Yep there it is thanks !

That's normal. That's always happened.

I'll try and see what happens. Cool. It worked.

Actually it appears just about anything works as long as it starts with "general.useragent". I'm currently set in FireFox about:config

string: general.useragent.extra.distro
value: Slackware

[EDIT_1]
If you're wondering how to delete an old string/value pair just "reset" the string. If it's an entry you created it should now have a blank value. Close FF and reopen and the entry should be gone.
[/EDIT_1]

[EDIT_2]
After a bit of googling around I found a few useragent test sites. It appears that there are standard entries for vendors to use for further identification. I've now added 2 new strings:

string: general.useragent.vendor
value: Slackware

string: general.useragent.vendorSub
value: 12.2_stable

More trivia to know!
[/EDIT_2]

I was curious if this can be done to Firefox on a Windows box...

*Edit*
It can be done...

I thinnk the Windows one also may involve a registry hack. WRT the logo, it doesn't show up until you refresh the page.

Just a test

Err, but what about Opera?... ;(