After reinstalling Zotero, or after switching between Zotero for Firefox and Zotero Standalone, you may see the following error message when you you try to open Zotero:

This version of Zotero is older than the version last used with your database. Please upgrade to the latest version from zotero.org.

This message results from trying to use a Zotero database from a later version of Zotero in an earlier version of the software. For example, if you previously installed Zotero 4.0.13 and later installed Zotero 4.0.12 into the same Firefox profile, or if you upgraded Zotero for Firefox to 4.0.13 but still had Zotero Standalone 4.0.11, you would get this error.

By far the most common cause of this error is having used Multilingual Zotero (MLZ). The current version of that software, now called juris-m, does not longer cause this. See this post by MLZ's developer on how to securely return to using regular Zotero.

Most Zotero versions preserve backward database compatibility, but occasionally it's necessary for Zotero developers to make changes to the database that prevent it from working with previous versions.

The best solution is generally to reinstall the latest version of Zotero from zotero.org. (If you were using a development build, you will need to install the latest dev XPI.)

If for some reason you don't want to or are not able to install the latest version, you can restore the pre-upgrade database backup that Zotero stored in your Zotero data directory. Look for the zotero.sqlite.*.bak file with the highest number (e.g., zotero.sqlite.77.bak). Close Zotero/Firefox, move your existing zotero.sqlite file out of the way, and then copy the numbered backup file into place as zotero.sqlite. Then restart Zotero.

If you don't have data you care about, or if you've fully synced your data and files with the Zotero server, you can keep your current Zotero version by closing Zotero/Firefox, deleting your Zotero data directory completely, and, optionally, syncing to restore your previous data.