Setup: IE default browser, FF37 preferred browser and configured in BROWSER env variable

Test:

import webbrowser
url = "http://localhost"
webbrowser.open(url + '?XDEBUG_SESSION_START=sublime.xdebug')

Behaviour:

1st run: FF opens -> ok
2nd run: FF opens new tab -> ok, in addition, IE opens -> WRONG

Here is a matrix of the test results depending on which browser is open upon starting the test script:

No browser open: FF opens tab (ok)
IE open: new tab in FF (ok), nothing in IE (ok)
FF open: new tab in FF (ok), new tab in in IE (NOT ok)
Both open: new tab in FF (ok), new tab in in IE (NOT ok)

Looking further in webbrowser.py, FF uses GenericBrowser:

class GenericBrowser(BaseBrowser):
    """Class for all browsers started with a command
       and without remote functionality."""

   ...

    def open(self, url, new=0, autoraise=True):
        cmdline = [self.name] + [arg.replace("%s", url)
                                 for arg in self.args]
        try:
            if sys.platform[:3] == 'win':
                p = subprocess.Popen(cmdline)
            else:
                p = subprocess.Popen(cmdline, close_fds=True)
            return not p.wait()

If FF is already open, Popen.wait() returns TRUE for FF but FALSE for IE. This leads to fallthrough for FF and then calling open for IE.

It boils down to Popen.wait() returning 1 if FF is already open resulting in GenericBrowser.open() returning False.

That part was last touched here https://github.com/python/cpython/commit/a456db5e058f955f235fe7a51e8c111d0a8ecf4e

And finally: FF returns exit code 1 when its started and already running. That situation is not handled by the GenericBrowser Popen logic.

Last but not least: this is due to using BROWSER for defining the executable path. 

Workaround: set BROWSER=firefox and add firefox to path.