Do you mean that you get these pages every time you start Firefox?

You can check for problems with preferences.

Delete possible user.js and numbered prefs-##.js files and rename/remove the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in current Firefox releases.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

"... instead it should display 'added to firefox', right?"

No. The Add-ons website can't 'see' which extensions a user has installed; nor does it track which extensions have been installed by each user from the Add-ons website.

I imagine a cookie could be set as part of each download / installation, but as soon as the user deletes their cookies that 'data' would be lost. Plus, these days their is a huge do not track movement going on to maintain privacy and limit tracking, of which Mozilla is a leader in user privacy. Therefore, using a cookie to 'track' extension installations would run counter to the trend of increasing user privacy.

Thank you for the answer. I appreciate it. But on my firefox, 'Do Not Track' is off. I noticed on chrome so, I just got a doubt about it and asked why it is not on firefox. I hope your answer to be one of the reasons for my question.

Firefox allows any website to check what Plugins you have (as long as they are not set to "Never Activate"). I have a test page which shows how this works here: https://jeffersonscher.com/res/plugins.html

But Firefox does NOT allow websites to check what Extensions you have installed unless you explicitly grant the site "remote troubleshooting" permission. This would be similar to getting a request to allow notifications or share your location, it would drop down from the icon to the left of the URL on the address bar. You wouldn't be able to miss it. I have been planning to create a demo page for that but have not gotten around to it.

I should note that if you use the "Get Add-ons" section of the Add-ons page, then Firefox sends a list of your current extensions to the site in the URL and you see a special view particular to that embedded frame on the Add-ons page. When you view the Add-ons site outside of the Add-ons page, Firefox doesn't send that information and you get the general public view of the site.