If you already have their add-on installed, uninstall it then re-install it.

Check your browser is up to date, along with all plugins:
https://www.mozilla.org/en-US/plugin...incheck-update

When MEGA first started, it didn't really use Flash, but when one day I set Flash to "Ask to activate" (click to use) and MEGA required Flash to be enabled. :/

If you're using anything like NoScript or other add-ons, whitelist the site as it may break functionality if you have it blocked.
There are no ads on MEGA or anything else so you shouldn't ever get any kind of malware warnings.

You can also use an external downloader:
Internet Download Manager, JDownloader, etc.

I personally use JDownlaoder 2 Beta.

Hope that helps.