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FirefoxCSS-Store
A collection site of Firefox userchrome themes, mostly from FirefoxCSS Reddit community.
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
Servo - https://servo.org/
Also for CSS, firefox has this and themes and extensions are readily available in firefox addon stores. But for plugins i don't think that they are even meaningful but still they are available in firefox. although you have to install that directly from the local files after you downloaded it from your source[s].
SerenityOS has its own browser made from the ground up. It has a long way to go though. You can goto https://www.youtube.com/c/AndreasKling/videos and look videos with "browser" in the title to see some of the work that goes into improving the browser.