Mine says "the release update channel" on Windows. I believe this helps you know whether Firefox is checking for new official releases or new beta test releases. I don't know what default update channel indicates.

A web search indicates that default means no updates. To change your update channel, check out this article:

app.update.channel - MozillaZine Knowledge Base

See also: Option to change update channel not appearing in Help>About. Why? | Firefox Support Forum | Firefox Help

Thanks for the great info, jscher2000.
Unfortunately I found the following in about:config:
app.update.channel = default
app.update.enabled = false
I changed them to release and true and restarted Firefox. When I open the About Firefox dialog it still shows "You are currently on the default update channel" (I tried this on both my user and root user).

Thanks anyway,
Adam.

Since bug 659972 the official policy for switching channels is / was to do a reinstall. See cor-el in question 849219. This was also published on Planet Mozilla back then. The documentation about this feature is mostly out of date.

Okay,
Although the app.update.channel can be found in about:config and this link suggests that is how you edit it (app.update.channel - MozillaZine Knowledge Base) a separate "Caveat" on the page indicates this preference can only be changed by "editing channel-prefs.js".
See additional information for updating this in channel-prefs.js:
Software Update

To summarize:

1. The update channel is configured when Firefox is compiled.
2. For most RPMs this is set to default (no updates) so the RPM manager can control software updates.
3. This configuration (app.update.channel) can be found in the about:config preferences but changing it there will not take effect.
4. The channel channel-prefs.js can be used to change this to release (standard released versions).
5. Making this change causes the About Firefox dialog to show you are on the release channel but it appears to still not perform updates =(
   

Edit: As commented by knorretje this bug report shows a dialogue of the developers related to this feature.
In mid 2011 the developers removed channel switching functionality from the user interface (UI) of Firefox 5 and 6. I believe this has left some non-UI traces of this feature with some issues that still need to be worked out. We may see some adjustments to this functionality in the next few builds.

At this time I will need to remove Firefox from the RPM manager and install and maintain it manually. In a lot of ways this makes sense because the RPM manager is responsible for keeping track of the version of software it manages. The only problem is you can't use the latest Firefox if you have RPM dependencies on it.

Adam.

Short answer for being on the default channel, it is due to using a third-party build from you Linux distro (also if one builds it themselves) as Mozilla does not provide updates to builds they did not well build.

If you use the Linux builds from Mozilla then you will be on the Release channel.

Since you are using a third-party build, even if you change the channel to one of the four choices you may still not get any updates.