Why is there no Zotero button in the Firefox toolbar after installing or upgrading Zotero for Firefox?
First, make sure you're looking in the right place. Zotero can be opened from a “Z” button in the main Firefox toolbar, the toolbar overflow panel (opened via a “»” button to the right of your other toolbar buttons), or the new Firefox menu panel (opened via the “Open menu” button with three horizontal lines at the right edge of the Firefox toolbar).
If you don't see Zotero buttons anywhere, make sure you have Firefox 45 or later. If so, make sure you have the latest version of Zotero installed and enabled by opening the Firefox Add-ons Manager via Tools → Add-ons or, on some Windows versions, by selecting Add-ons from the main Firefox menu. Zotero should be listed in the Extensions pane of the Add-ons Manager, and you should have version 220.127.116.11 or later.
If you don't see Zotero in the Extensions list or you don't have the latest version, install it from zotero.org. You should have the latest versions of both Zotero and Firefox.
If Zotero is listed as installed but you don't see the Zotero button in the three places listed above, open the menu panel by clicking the “Open menu” button in the Firefox toolbar (the icon with three horizontal lines) and click Customize. You should see multiple Zotero buttons in either the customization page to the left or the menu panel to the right. You can choose between a combo button, which includes the “Z” to open the Zotero pane and a save button to save from any webpage, or separate buttons for opening and saving to Zotero that you can place in separate places. (See the 4.0.27 announcement for more information.) Drag the button of your choice where you want them. If you only open Zotero infrequently and would prefer not to take up room in your toolbar, you can keep both buttons in the menu panel and use the “Open menu” button to access them.
If Zotero is listed in Add-ons → Extensions but you don't see its buttons even in Customize mode, you see the message “This add-on will be installed when Firefox is restarted” even after restarting, and/or you get other errors during installation, try the steps under Corrupt extension files on the Mozilla add-ons troubleshooting page.
You should also try disabling all other Firefox extensions, as some are known to cause problems with Zotero.
After a Firefox Reset
If Zotero disappeared because you performed a Firefox reset, your Zotero data will still be missing after you reinstall the extension. See Restore After Firefox Reset for instructions for restoring your data.