Install Older Version Of Firefox {web link} Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release {web link} ESR Notes System Requirements

Thanks Fred. I still don't know what the .dmg extension (Firefox52.1esr.dmg) is and what I do with it. The site I went to said to download it to fix my problem but I can't do anything with it.

A .dmg file is a program installer for Mac computers. I would step back from whatever site told you to install that as it doesn't sound trustworthy. Or if you want to give a link to volunteers can see what's going on, feel free to post one.

Thanks for the help. The file came from the Charles Schwab website. I'll call them.

Ah.

If they are suggesting you downgrade Firefox so you can run the Java plugin or another old plugin, please see this article for more background: Why do Java, Silverlight, Adobe Acrobat and other plugins no longer work?