Sounds like one of your add-ons is misbehaving. You can try to isolate which one by disabling them one at a time and restarting Firefox. Please let us know which one it is so we can inform other users. Thanks.
User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:7.0.1) Gecko/20100101 Firefox/7.0.1 Build ID: 20110928134238 Steps to reproduce: Opening a new window Actual results: Opening a new window often shows nothing. Sometimes then I find a REALLY small vertical line. Sometimes I succeed to stretch this line to a readable window. Most times I have to restart Mozilla. Was the window for download opened it reopens and the (then 2nd) new windows doesn't appear. I have to close the window for downloads and restart. VERY irritating Expected results: clear. The new window should appear
Well I tested add-on per add-on but could not find anyone. It seems to be a problem especially to the download window....
Can you try starting in Firefox Safe Mode just to make sure? http://support.mozilla.org/en-US/kb/Safe%20Mode (don't click any of the check-boxes, just "Continue in Safe Mode")
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