First impressions count; that is true for everything including web browsers. When it comes to web browsers, startup performance is important. If it takes (a lot) longer than the previously used browser, users may be inclined to go back to the other browser.
Mozilla Firefox loads the about:home page by default. The page resembles the browser's new tab page, but is handled differently by the browser. The web browser displays top sites, highlights (visited sites, recent bookmarks or downloads) and, recommendations by Pocket on the page.
Firefox users may customize the page, e.g. by pinning sites to the list of top sites, or removing entries from the highlights section.
Most Firefox users that I know enable the browser's "restore previous session" functionality or configure the browser to load one or multiple specific webpages on start.
Those who don't, get about:home any time the browser is started.
Mozilla engineers started to investigate options to improve the loading performance of this essential page. A new blog post by Mike Conley provides all the details in case you are interested in technical implementation details.
Basically, what Firefox does is cache the about:home page so that it loads faster on consecutive runs. Mozilla's own tests showed a startup performance improvement of about 20%. Conley published a side-by-side video to demonstrate the improvement.
YT video inside the source
Mozilla engineers implemented a caching system for the browser's startpage to improve the loading performance of the web browser on start.