What is the current behavior ?
Blank Popup
If the current behavior is a bug, please provide the steps to reproduce and if possible a demo of the problem.
Install addon. Close firefox. Open firefox. Navigate to site. Select wappalyzer tech icon in address bar to open popup. Popup is blank.

What is the expected behavior ?
Popup to not be blank and show other tech icons.
Which versions of Wappalyzer, and which browser / OS are affected by this issue ? Did this work in previous versions ? Wappalyzer 5.7.3 / Firefox 65.0.1 / Windows 10 - Worked in earlier versions. Not sure how long it hasn't worked for. Addon may have been updated a few times before I noticed. However, I have uninstalled and reinstalled. Restarting firefox at each step.
Navigate to what site?
any site.
I can't reproduce this :/
Do you have any errors in the console?
There are errors in the debug console to do with firstPartyDomain as per image.

+1 for this issue.
Firefox 65.0.1
Linux(Debian based)
Wappalyzer 5.7.3
First party isolation enabled.
Debug log:

Edit:
Based on what is described here about first party isolation regarding Firefox my uneducated guess is that the culprit for this specific issue could be found in driver.js at line 151:
const cookies = await browser.cookies.getAll({ domain:.${url.hostname}});
Solution could be as simple adding to the details object the firstPartyDomain string in similar fashion as the domain string is passed. Of course take this with a grain of salt as this guess is based on quick read about the issue on developer.mozilla and a search on the sources.
I have reproduced this on another device: Wappalyzer 5.7.3 / Firefox 65.0.1 / Windows 10
@mhtmht comment reminds me that I don't run standard settings and have changed many in about:config as per Firefox: Privacy Related "about:config" Tweaks
I have being using settings from privacytools.io for a while now and wappalyzer had no issues up until recently. Hope the extra info helps.
Edit: I had an old vm with the privacytools firefox settings and an older version of wappalyzer so loaded it up and checked it out. Seems everything works fine with version 5.5.3 / Firefox 65.0.1 / Windows 10
@SabeBarker I also had this issue using the about:config settings from privacytools.io.
I "fixed" it by setting privacy.firstparty.isolate back to the default value of false. From what I understand, that setting prevents data from being sent to wappalyzer.com for purposes of enumerating the different web technologies being used.
Yes this is the issue. I now use First Party Isolation firefox extension to toggle this setting when I want to use wappalyzer. Works for me.
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Most helpful comment
I have reproduced this on another device: Wappalyzer 5.7.3 / Firefox 65.0.1 / Windows 10
@mhtmht comment reminds me that I don't run standard settings and have changed many in about:config as per Firefox: Privacy Related "about:config" Tweaks
I have being using settings from privacytools.io for a while now and wappalyzer had no issues up until recently. Hope the extra info helps.
Edit: I had an old vm with the privacytools firefox settings and an older version of wappalyzer so loaded it up and checked it out. Seems everything works fine with version 5.5.3 / Firefox 65.0.1 / Windows 10