Simply click on the icon to rename the tab title. Features:

Persistent throughout multiple sessions!
Titles saved across devices (under the same account)!
Domain-level titles!
Option to set a temporary title!
Option to lock the title to a tab! As long as you don't close the tab, the title remains the same.
Keyboard shortcut with Alt + SHIFT + X
Context menu option!
Use saved bookmarks' titles!
Prefix the original titles! (Using $0 inserts the old title)

Beta features: (Might not work perfectly)

Support Discussion: https://discourse.mozilla.org/t/support-tab-re-title-general-support/28395 Known Issues:

The old title replacement $0 breaks when the title is changed by the website. e.g. Getting a message on Facebook. Temporary Fix: Don't use $0 for websites that updates titles.
[Windows] The popup won't auto focus when clicked. This is a Firefox bug. Temporary fix: https://bugzilla.mozilla.org/show_bug.cgi?id=1324255
[For FF60 and up] PDF pages cannot change titles, due to firefox not allowing add-ons to interact with the PDF viewer. Temporary fix:Use FF 59 or lower, or use Google Chrome as it can still change PDF pages.

IMPORTANT NOTE: This doesn't work on *mozilla.org* websites, reader view, and PDF pages as FIrefox disables add-ons for security reasons.