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Firefox is on the brink of being dropped by the US Government
(www.brycewray.com)
This is a most excellent place for technology news and articles.
The FF extension is just called "User-Agent Switcher" I believe?
And yes, you can set it blacklist mode or whitelist mode (in other words, "use the extension on every domain but:" vs. "Only use the extension with these domains:").
I'll only switch away from FF if a site is completely broken. I'll try it before resorting to chromium