One of Mega's distinguishing characteristics is that, until now, it's been web-only. In fact, it's been Chrome-only, the site only works properly in Google's browser. Mega explained that some of the more advanced web technologies it uses aren't supported or don't work that well in other browsers.
Now though, Firefox users are getting some support in the form of an official add-on. The add-on is offered on the site itself to any logged-in user running Firefox.
Users get a prompt encouraging them to install the extension to get the most out of the site. The extension doesn't do much after it's installed, there's no UI or anything.
But it will allow users to download files of any size and grab several of them at a time, useful if you need several of your big files and fast.
The add-on completely bypasses Firefox's download process, it even saves files in the default Downloads folder rather than whatever you have set in the browser.
In Chrome, Mega downloads the file you need, the encrypted version, decrypts it locally and provides you with the original version.
This means that the encrypted file has to be "downloaded" into RAM and then decrypted there as well. Memory usage peaks at twice the file size, so a large file would quickly use up quite a lot of RAM, a DVD image would need more than 8 GB of RAM for example.
Note though that you can use Mega without the extension, downloads will just be slower and you will only be able to download one file at a time.