A couple months ago finally moved from 28 to 51 and was so disappointed at the appearance and some things lost I had to use Classic Theme Restorer just to make it look like Firefox again. I had to change some older plugin's rdf file to claim it would work on newer version and reinstall. I lost one that wouldn't work anyway, Elite Screen Capture. I do wish Mozilla would update the add-ons in earlier versions to the one they currently offer. I realize the add-ons are often done by others, but loss of favorite add-ons discourages updating FF.
For those users of Firefox, version 52.0 has been released. If you've got FF users settings enabled for automatic updates, nothing needs to be done. It will update automatically. If the browser is not set up to update automatically, then simply click on "Help" in the upper left, then "About Firefox", then "Check for Updates".
On the other hand, if you'd like a new, offline installer file of the browser, click on the link below and download the language and operating system version of your choice.
Release notes for Firefox 52.0:
Hope this helps.