I've recently been experiencing annoying delays when moving tab to tab as well as when closing tabs. I've tried restarting without add-ons with no apparent help.
You can check for problems with the sessionstore.js and sessionstore.bak files in the Firefox Profile Folder that store session data.
Delete the sessionstore.js file and possible sessionstore-##.js files with a number and sessionstore.bak in the Firefox Profile Folder.
Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost, so you will have to create them again (make a note or bookmark them).
You can also try to remove (some) history in case you keep a large history.