scriptcat

ScriptCat is a browser extension that runs userscripts: small bits of JavaScript that change how websites look or behave, like hiding clutter or adding a download button. It's an open source answer to Tampermonkey, which went proprietary years ago, and it runs Tampermonkey scripts. GPLv3, free on Chrome, Edge and Firefox.

Two things set it apart. Background scripts run without any page open, and scheduled scripts run on a timer, so a userscript can act like a small cron job living in your browser. Sync goes through your own storage (Google Drive, OneDrive, Dropbox, S3 or WebDAV), not a ScriptCat account. It also has a built-in editor with linting.

Anyone already on Tampermonkey who wants an open source manager: switch. Violentmonkey is the other open option, MIT licensed. There's no Safari build, so Safari users need something else.

The catch is the permission you're granting. A userscript manager can run code on every site you visit, and background scripts extend that to times you aren't looking. Only install scripts you've read or trust.