Whatever you would use a browser extension to do now, there was probably a standalone application to do the same thing before.

The original vision of the web didn't just have "sites with servers" and "consumers with browsers", but rather tons of potential independent "User-Agents" that would perform tasks on behalf of people. The consolidation of the web into just browsers, and subsequent flipflopping between creation and destruction (similar to the EEE cycle MS has employed), has killed whatever of that vision might have flourished.