Browser extensions are genuinely useful - they block ads, manage passwords, capture notes, check spelling, save money at checkout - and they install in seconds with a single click. That very ease is where the risk hides. In the moment of installing, the browser usually shows what the extension is asking permission to do, and an unnervingly common request is the ability to read and change everything on every website you visit. Most people click past this without reading it, because clicking past prompts is what one learns to do, and in doing so hand a small piece of software from an unknown author broad access to their entire browsing life.
The reason this matters more than an equivalent permission on, say, a standalone app is that the browser is where the sensitive things happen. It is where you log into your bank, your email, your business systems; where you type passwords and read private messages and enter card details. An extension with permission to read and change everything on every site can, in principle, see all of that - the pages you view, the things you type, the accounts you are logged into. A well-behaved extension uses that access only for its stated purpose. But the access itself is broad, and it is only as trustworthy as the author behind it and the security of the account through which the extension is updated.
The particular danger is that an extension can be safe when installed and become dangerous later. An add-on with wide permissions receives updates automatically, and those updates run with the same broad access already granted. An extension can change hands - sold to a new owner - or have its developer's account compromised, and a later update can then quietly turn a once-useful tool into something that harvests data or injects unwanted content, all without the user doing anything or being asked again. Because the permission was granted once and persists, the trust decision made at install time silently covers every future version, including ones written by someone else entirely.
The sensible discipline is modest and mostly about paying attention at the moment of installing, then occasionally afterwards. Before adding an extension, read what it is asking for and ask whether the request fits the job - a tool that only needs to work on one site has no obvious reason to read every site. Prefer extensions with a clear, reputable author and a real user base over obscure ones offering the same trick. Install only what you actually use, and periodically review the list already in the browser, removing the ones you have forgotten about, because each is a standing grant of access that persists whether or not you still benefit from it. The browser is the most sensitive application most people run, and a moment's attention to what its add-ons are allowed to see is a small price for keeping that access in trustworthy hands.