Default Zoom от Jamie Nguyen

Set the default zoom level for Firefox. Useful for QHD or HiDPI monitors, or if you don't like squinting.

• Set your desired default zoom level. If unset, the add-on will default to 100%.
• Per-site zoom is respected (with one exception, see below).

It's Free and Open Source software and you can view or adapt the source code.

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Other zoom add-ons have more features and might suit you better.

This add-on makes some trade-offs to be as tiny as possible. It's just 30 lines of JavaScript (whereas others are 1000s of lines). It doesn't use browser storage other than a single local option for the default zoom level, and asks for no permissions other than to store that option.

Per-site zoom levels are respected, with one exception: a zoom of 100% is always overridden after you reload a page. (Reason: There's a limitation in the WebExtension API. Other add-ons solve this by storing per-site zoom level entries for every website you visit. This add-on avoids storing those entries by only zooming when the zoom of the current tab is 100%, which is fine because 100% was too small for you anyway!)