You could either copy the profile or use Firefox Sync to do this.

So, just so I'm sure, if (1) I download and install Firefox onto my new computer and then (2) I copy my old profile (from my retired Windows 7 computer) into the correct location on that new Windows 10 computer, I'll have all my old bookmarks and saved passwords? It's actually that simple?

(I'm not sure how Firefox Sync even works, but I'm guessing it's not a solution since I'll be unplugging and moving my old desktop computer out of the way to make room for a new desktop, but thanks for the suggestion.)

Firefox Sync is supposed to work well when upgrading to a new PC, but now is not the time to learn how to use it properly and successfully, IMO. One simple mistake with your account password and all your data could be gone from the Sync server.

See these support articles for doing a "backup" and "restoration" of your Firefox user data files: https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

I have partitioned my computer to have Windows 7 and Windows 10, while I am making the transition to 10. I have confirmed I am signed in to my Firefox account on both OS's, but in Windows 10 it's showing bookmarks from over a year ago. It's not syncing with my current bookmarks. I do not understand how or why the sync isn't happening.