The explanation MEGA gave a while back for it sucking in Firefox is that Firefox doesn't support writing files via HTML5 FileSystem API. Of course, I agree that using JavaScript to download files is not a very good experience. In Chrome, it doesn't download it to RAM but to the disk using the HTML5 API, but the experience in the web browser still isn't great unless you have a fast machine.
The MEGA client on the other hand isn't bad though - it's lightweight, fast, not updated constantly, it downloads extremely fast and the context menu verbs it adds disappear without bloating the context menu once the client is exited. I have MEGA accounts of 50 GB from when they were giving away huge storage for free and I guess plenty of people grabbed them. That's why it's somewhat popular.
For that matter, there's also a long-standing bug in Google Chrome when downloading big files from their own Google Drive which causes huge file transfers to fail abruptly. Strangely when using Firefox to download from Google Drive, the bug doesn't exist. OneDrive is also a good alternative, even for that, I grabbed 25 GB accounts when they were giving those. But then I trust big brother Microsoft less than big brother Google. That's why Mega. Dropbox's free accounts suck IMHO. I don't know any other reliable service still around for years offering big storage in free accounts.
As for download speeds, MEGA gives some impressive speeds. But so does OneDrive or Google Drive due to their distributed network of datacenters all across the globe. Just my two cents.