UPDATE: If you're having trouble installing from the wiki page, try right-clicking the link to "Save the link as..." (ctrl-click under Mac OS X), then install from your desktop by dragging the file and dropping it on a Firefox window. For more information on extension installation failures, check out Mozilla's page on the topic.
As for the server, we're making continuous strides on improving efficiency and performance. A change we completed this evening improves overall throughput substantially; this will translate into fewer "Server is unavailable" messages during our peak hours.
Version 2.0.38 also has a role in this equation. Previous versions of the extension had a not-so-well conceived backoff strategy if it found the server was busy: it would try again in two minutes. This is like everyone hitting redial on their phone trying to get through to the radio station that's giving away Celine Dion tickets: it just clogs the circuits so that no one gets through. By being less aggressive (some might even say less obnoxious) in its backoff strategy, this new version of the extension will allow more people will be able to get through in a more orderly fashion.
This last bit won't yield visible benefits until the new version is in wide use. So if all goes well, we'll be making this update available on the Mozilla Add-ons site for general distribution later this week.
If you see something strange in this new version that you'd like to alert us about, please feel free to post here. If you have a problem that requires a response, please send email to support.