Tab warming is a fairly new concept to Firefox browser. It can be simply explained as preloading a webpage when you are switching tabs.
The intention by Mozilla with Tab Warming is to speed up tab switching process by improving the performance of switch from one tab to the next. Usually, Firefox does a decent job when it comes to tab switching, but some websites do have some noticeable delays when you are switching to them from another tab on Firefox browser.
Particularly pages using SVGs and CSS for animation, they tend to have a noticeable lag when you switch to them from another tab. As it stands, when you are viewing one tab and move your cursor to point at the next tab, Firefox will begin to render the page in the background before you click the tab.
By doing so, Firefox gains the time it takes to load up the webpage, as it began loading the page in the background the second you moved the cursor to that tab before clicking on it. Mozilla explains it as follows:
With tab warming enabled, when you hover over the Click here to read entire article