User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:66.0) Gecko/20100101 Firefox/66.0

Steps to reproduce:

Installed an older version (66) temporarily to workaround a bug.

Actual results:

Firefox keeps updating itself even though in the settings I had set it to NOT automatically update, but only notify me. I will update anyway, and even reset that setting to auto-update.

Expected results:

It should respect the settings I make for not auto-updating. There's a reason I set that, and Firefox should know its place not to question my decision, especially not without consulting me first.

It's fortunately finally fixed in Firefox 69.

Okay good, but how do I keep Firefox at 66 for the time being?

If you have administrator access, see bug 1420514, comment 137.