They can be a bit of a safety risk!
The browser is the primary destination for most users. Sign up for a specific service, and you can do most of the things required in the world of work and play. For every esoteric functionality, there probably is a browser extension. Large companies make some extensions, and these tend to add an improved experience to the core web service.
Most extensions are developed by individuals with an open-source model of transparency. Successful extensions may eventually have millions of downloads, but managing them may take resources that the original developer does not have. The question of monetisation comes up. Some grapple with ads or they sell the extension.
The recent examples of Great Suspender utility for Chrome and ModHeader focus on the security and safety issues that have cropped up. Code injection, unauthorised ads and even data exfiltration are some of the immediate concerns.
Suggestion: Check the extensions installed in your browser and evaluate if you need them all. Remove the ones that are superfluous and be very circumspect about installing news ones too.