You can see the title if you pass the mouse on the tab:

• Tips Pinned tab

I also found this old questions, maybe help you:

• questions/964352
• questions/930273

That last link has a custom style rule you can use to give pinned tabs the same 250 pixel maximum width as other tabs. If that's wider than you need, you could edit the rule to your preference.

that is the way pinned tabs work.
You only see the favicon and not the text.
To see the text on tabs you have have to use normal tabs and reduce its width to a lower minimum.

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#tabbrowser-tabs .tabbrowser-tab:not([pinned]) { max-width:20px!important; }

You can also consider to hide the favicon to see more text.

#tabbrowser-tabs .tab-icon-image { display:none!important; }

you can delete the storage\moz-safe-about+home folder in the Firefox profile folder to reset snippets stored in IndexedDB.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)