hello, you're right to suspicious about this, since it is a scam tactic that is trying to trick you into installing malware, so don't download or execute this kind of stuff! only download firefox from its official location at mozilla.org - in addition updates are handled automatically by firefox.

you could also help reporting such pages use the firefox logos or trademarks to trick users into downloading malware at https://www.mozilla.org/en-US/legal/fraud-report/ for mozilla to take further actions on it.

i'm not sure how the fake site got displayed on your device, but just to be sure, please go into the firefox addons manager > extensions panel and remove any suspicious entries (toolbars, things that you have not installed intentionally, don't know what purpose they serve, etc). Troubleshoot Firefox issues caused by malware

That is a Fake site claiming Firefox updates to get inexperienced Windows users to download malware.

Firefox updates will be in Firefox itself or by mozilla.org

Firefox 39.0 was only just released late Thursday last week. If there was a update it would be 39.0.1

Note that in this case a .exe installer is offered that will only work on the Windows platform.

If you get a pop-up message or web page asking to update Firefox or plugins or scanning for malware then such a message is likely a scam and you should never respond to such an alert to avoid getting infected with malware.

• Only update Firefox via "Help > About" or by downloading and installing Firefox from the Mozilla server and never via a pop-up or link on a web page.
• Plugins should only be updated via the plugin itself or by visiting the home page of the plugin.