I don't think any of the multiprocess code was rolled out as far back as 45.* (could be wrong).
If it is, you should be able to check in the about:support page. If it's present but not active, then as someone else said, it will be because you have an add-on installed that's not compatible with it.
To see which addons are and aren't compatible with multiprocess, you can install a Mozilla add-on called Add-on Compatibility Reporter; once installed, this will add (in-)compatibility labels to each installed add-on's listing in the add-ons screen.
Sadly, if you've been collecting add-ons for a while, you'll probably find quite a few marked incompatible. I'm using quite a few that are pretty indispensable, both at home and work, and which all evidence suggests are unlikely to be ported to WebExtensions* as the developers have long-since moved on. It will be a real wrench and I'm honestly not sure my fondness for the Fox can endure it; I fear the add-on catalogue will be a shadow of its former self soon, and with that goes a lot of what made it awesome. Sad times.
[* Because it's not just a question of replumbing extensions to work with multiprocess, oh no. The whole extension architecture is being thrown out and replaced with something [semi-]compatible with the WebExtension format as used by Chrome and Edge. So far, I've seen nothing to suggest that the possible gain of ease of porting addons from those browsers will outweigh the loss of the existing massive and diverse AMO collection.]