Measuremate is a browser extension for Mozilla Firefox, which appears on the toolbar (besides the search bar) if you allow it to show there. You just have to click on the icon and the extension will appear. You just have to type in 1 input and all the others will be automatically converted. The conversions are not rounded for exact precision as allowed by the web language javascript. The UI is friendly and easy to use.
The extension is currently on v1.5 and a feature you have never seen in other convertors is that the feet and inches are separate inputs but act as one. Suppose you want to convert your height from feet and inches to meters, other convertors will ask it in decimals, whereas, Measuremate has separate feet and inches so you don't have to go to another calculator app and convert feet and inches to decimal feet. The extension makes it easy for people to convert with exact precision.
Measuremate is currently only available for desktop Firefox Mozilla and is open-source. It requires the permission to access your data for all websites. It does not break any privacy code, and this permission is the main reason the extension is source code, so people can always check what the extension is actually doing. There is no hidden code, all is right in front of you.