Why is my browser using so much memory?
Modern browsers deliberately trade memory for speed and stability, and a high figure is not automatically a problem. Each tab, and often each extension, runs in its own process so that one misbehaving page cannot take down the browser or read another site data. That isolation is a genuine security feature and it costs memory.
The consequence is that memory scales with what you have open. Dozens of tabs, particularly ones running video, chat or dashboards that update continuously, will consume a great deal. A page left open for days can also accumulate memory through its own leaks, entirely outside the browser control.
Extensions deserve specific attention because they are frequently the largest consumer and the least visible. Every extension runs continuously, some inject scripts into every page you visit, and a poorly written one can consume more than the pages themselves. Browsers include their own task manager listing memory by tab and extension, which identifies the culprit in seconds and is far more useful than guessing.
If the machine has ample memory, unused capacity provides no benefit, and the browser releasing it slowly is reasonable behaviour. The problem is only real when the system starts swapping to disk, at which point everything becomes slow.
The effective remedies, in order, are auditing extensions and removing what you do not use, enabling the tab sleeping feature most browsers now offer to suspend background tabs, and closing or bookmarking tabs you are keeping only as a reading list.
Restarting the browser occasionally is a legitimate fix rather than an admission of defeat, since it clears accumulated state from long-lived pages.