When trying to open Firefox I get this message--A copy of Firefox is already open. Only one copy of Firefox can be open at a time.

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Newly installed on new computer.
Create a new profile as a test to check if your current profile is causing the problems.
See "Basic Troubleshooting: Make a new profile":
If that new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.
See also:
* https://support.mozilla.com/kb/Firefox+is+already+running+but+is+not+responding
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