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I was able to duplicate this, but it's an issue with Firefox, not
Python. webbrowser.open(url) just passes url as a command line argument
to the web browser; it doesn't do any manipulation.
Note that you get the exact same behaviour if you run Firefox from the
command line:
> firefox 'http://foo.com/bar.html?var=x|y|z'
Opens this URL in a new tab if it's already open, but splits on '|' and
opens in 3 separate tabs if Firefox isn't running.
Note also that while this string is a URL, it isn't properly normalized.
This works fine if you call
webbrowser.open("http://foo.com/bar.html?var=x%7Cy%7Cz")
(Which you can only obtain programmatically by generating the URL
properly in the first place, by using urllib.urlencode, or urllib.quote
on the value string "x|y|z").