No luck involved.
1. Go here: https://github.com/AdguardTeam/AdguardBrowserExtension/releases/tag/v2.7.2
2. Scroll down to the downloads and find firefox-standalone-beta-2.7.2.xpi (it says beta, but it's a final version - the standalone version is "enhanced" with slightly better blocking abilities due to the restrictions of AMO).
3. Right click on it and save the .xpi file somewhere you can find it (e.g. the desktop).
4. Go to Firefox's add-ons (about:addons) go to the Extensions section, click the settings cog and select Install Add-on From File... and find the .xpi you downloaded and install it.
Just tried this with a clean Firefox 57 profile several times and it works 100% of the time for me. Maybe that's the key, a clean profile?
Just a random thought, are you using any antivirus/antimalware software? If so, what? Also, what Windows are you running?
Also does the Assistant fail to appear on specific sites? If so what? Or does it straight up fail on all sites? If it's the latter, chances are the devs will need logs - at the very least you can open the browser's console and see if there's any errors there.