Tor Browser, apart from Firefox, also consists of multiple components like pluggable transports.
With the operating system vendors long dropping support for these legacy operating systems, even even with Firefox extending support, it has become unfeasible to maintain the browser as a whole.
Tor Browser 13.5.29 was the final update through the legacy channel. Users on Windows 7, 8, 8.1 and macOS 10.12, 10.13, 10.14 are advised to upgrade their operating systems for access to the up-to-date version of Tor Browser.