Hi   !
This is a fake   ! Firefox will always update from within the browser and not from a random webpage. Scammers use this tactic, trying to get users to download and infect their computer. Hard to get rid off, as they will change their URL on a daily basis. You could consider an add-on blocking extension, such as:
http://addons.mozilla.org/en-US/addon/ublock-origin
Would you also take a look at the article:
'Update Firefox to the latest version':   http://mzl.la/152VFwM

Thank you for reporting this   ! Please see the article:     I found a fake Firefox update.

Hopefully you did not download or run the malware.

If you downloaded it (but didn't run it), please delete the download very carefully as follows:

When you are using the downloads panel (the one attached to the toolbar button), be careful not to click anything as that may run it. Instead, right-click it and choose Open Containing Folder. That will launch a file window with the unwanted download highlighted, and then you can press the Delete key to send it to the Windows Recycle Bin.

If the download has already disappeared from the panel, the same mouse action works in the full download list (Ctrl+j or "Show All Downloads").

If you already ran it, please delete the download and also use the free cleaning tools in our support article: Troubleshoot Firefox issues caused by malware.