The selected data directory is too old to be used with this version of Zotero.
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So I downloaded and installed Firefox (I use Chrome). Then I went to https://www.zotero.org/download and tried to install Zotero 4.0 for Firefox. I got an error message: "www.zotero.org The add-on downloaded from this site could not be installed because it appears to be corrupt." Also, I'd like to use an alias and not have my real name visible in the forum. Thanks.
You'd get this error if you were last using an extremely old version of Zotero (like, 6 years old, if not more) and then upgraded to Zotero 5. If you don't think that's the case, it's possible something happened to your active Zotero data directory and Zotero ended up finding a very old data directory elsewhere on your system. You would want Zotero Standalone 4.0, not Zotero 4.0 for Firefox, but first it would be good to confirm that you were actually using a very old version of Zotero. If you don't think that's the case, you would want to check the current data directory location in the Advanced → Files and Folders pane of the Zotero preferences and look for other zotero.sqlite files elsewhere on your system to see if somehow the connection to your previous data directory was lost. You can change that from your account settings. When you go to change your username, there's an option to use a different name for the forums.
So then, yes, this is expected coming from a very old database, and you need to open the database in Zotero Standalone 4.0 first to upgrade it before installing Zotero 5.