Edge should already have been there as standard if you're using Windows 10.
If you're using Windows 7 it seems that, despite officially ending support for the operating system earlier in the year, Microsoft has been pushing out an update that automatically installs Edge onto Windows 7 computers. They're clearly annoyed that only around 10% of people using their operating systems also use their own browsers (Edge or Internet Explorer), with around 70% of users preferring Chrome (from Microsoft's biggest rival, Google) and with independent browsers, such as Firefox (8%), still managing to maintain a decent foothold in the market.
It's all about money, of course, as Edge uses Microsoft-owned Bing as its default search engine, whereas most other browsers use Google; the more people Microsoft can get using Bing, the more money they'll make from advertising.
If Firefox is running slow for you, simply reset it to its 'out of the box' state. (You won't lose any bookmarks or log-in details by doing so). Click on the three lines (top right), then on Help. From there, select Troubleshooting Information. Once there, click on Refresh Firefox (top right).