Is there a way to find out how long a tab has been open, or when a tab opened?

In Firefox:

• History
• Show All History
• Views
• Show Columns
• Most Recent Visit

Works on all open tabs and tabs that have been closed, as long as you don't clear history often.

See my answer here. Basically, use this bookmarklet:

javascript:alert(new Date(performance.timing.navigationStart).toString())

Or more specifically, as a duration (using a combination of answers from this question):

javascript:var d = new Date().getTime() - performance.timing.navigationStart; alert(Math.floor(d/(24*60*60*1000))+" days, "+new Date(d).toISOString().substr(11,8))

Type the 'j' and paste the rest into the address bar, as browser security measures automatically remove the javascript: prefix, and might, if you then press enter, (annoyingly) turn it into a search, which will navigate away from the target page and make it useless.