My advice is to add Mozilla's apt repo for Firefox by following the handy guide on their website. It's pretty short and easy (copy and paste).
You can limit the file system permissions of the app, like giving only access to downloads, so that if/when there’s a sandbox leak you’re fine. You can also disable various things, like webcam or mic, this way.
In addition, you can get perpetual updates to the latest version of your browser even on old, stable distros like Debian.