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github-wc-polyfill
Ensure that all GitHub and GitLab scripts required for UXP and SeaMonkey are loaded correctly
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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
Github is also crappy, but can be made to work pretty much perfectly with the help of an add-on called 'GitHub Web Components Polyfill' (note that you'll have to extract it, edit its install.rdf to report that it's compatible with this version of Seamonkey, since it only claims to be compatible with more recent versions, and then zip it and rename it back to a .xpi file if you want to install it).