I'm going to assume that the issue doesn't happen when you run Firefox in Safe Mode?

In that case, one of your add-ons is modifying the user agent string in Firefox. The user agent in the information that's given to a website about what browser you are using. If you modify this, the Firefox add-ons website won't be able to tell that you are using Firefox, so it will tell you to install it.

If the issue still happens in Safe Mode, check your security software's settings. Some security/privacy programs have this "feature" and you will need to disable it in there.

They will often modify it because it "improves your privacy", but in reality (it more often than not) breaks functionality on websites.

Hope this helps.

Wesley Branton said

I'm going to assume that the issue doesn't happen when you run Firefox in Safe Mode? In that case, one of your add-ons is modifying the user agent string in Firefox. The user agent in the information that's given to a website about what browser you are using. If you modify this, the Firefox add-ons website won't be able to tell that you are using Firefox, so it will tell you to install it. If the issue still happens in Safe Mode, check your security software's settings. Some security/privacy programs have this "feature" and you will need to disable it in there. They will often modify it because it "improves your privacy", but in reality (it more often than not) breaks functionality on websites. Hope this helps.

thx. i think, it's security program problem. (symantec endpoint) and i have no authority. (company policy) ioi

You can try to reset the useragent to the default.

https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox