[corin_]

Hmm, might be tough - I'm not sure quite how Nightly releases work, but I updated an hour ago and the version changed from 18.0a1 (old date, a few days ago I think) to 18.0a1 (2012-09-30). Meanwhile I check my useragent and I'm just seeing Mozilla/5.0 (Windows NT 6.1; WOW64; rv:18.0) Gecko/18.0 Firefox/18.0

Will submit a pull request if I get round to looking further and find a decent answer!

edit: Not sure there's going to be any good solution. 18.0 Nightly was first out on August 28th, which is 33 days ago, so despite the fact that I last updated today, and that the whole concept of "Nightly" is to have an update every day, the release number is technically older than the 25 days since the most recent stable release came out. And when, in 10 days, 18.0 moves to Aurora and Nightly moves to 19.0, the user agent most likely won't be different between Aurora users then and Nightly users now.