Kind of a big update... this is the last version of Firefox for Windows 7 and Windows 8, along with macOS 10.12, 10.13, and 10.14. I know this is forcing family members to finally upgrade their (mac) OS.
More big updates:
Hardware video decoding is now enabled for Intel GPUs on Linux.
We've refreshed and streamlined the user interface for importing data in from other browsers.
The builtin editor now behaves similarly to other browsers with contenteditable and designMode when splitting a node, e.g. typing Enter to split a paragraph, and also when joining two nodes, e.g. typing Backspace at the start of a paragraph to join the paragraph and the previous one.
Pretty sure that last feature is what finally fixes the Reddit Fancy Pants editor in Firefox... exactly when Reddit is destroying itself.
IndexedDB is now also supported in private browsing without memory limits thanks to encrypted storage on disk. The temporary keys to decrypt the information are hold in RAM only and all stored information is purged at the normal end of a private browsing session from disk.
This might help WhatsApp web run in private browsing, among other sites.
The most impactful thing is definitely going to be OS compatibility... this is going to be the last version of Firefox for a long time for a lot of people.