Youtube V4 Browser Hot -

Could you please explain:

YouTube's transition from its early versions to the current iteration represents a shift from simplicity to data-intensive complexity. The "v4" era is often remembered for its —elements that look like real-world objects—and its snappy performance on low-end hardware. Modern browsers often struggle with the current YouTube site because of heavy JavaScript and high-resolution video codecs, making the "hot" or popular trend of using v4-styled browsers a practical choice for aging devices. Why "v4" Styles are "Hot" Right Now youtube v4 browser hot

The magic string often looks like this: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:25.0) Gecko/20150101 Firefox/25.0 Could you please explain: YouTube's transition from its