PMRes Viewer
Installer: https://uploads.realityripple.com/Projects/PRV_Setup.exe
Portable: https://uploads.realityripple.com/Projects/PRV_Portable.zip -- Protip: Use the portable version to avoid the installer adding my in-house code signing certificate to your trusted certs.
This will not let you build your own archives, only extract existing ones.
There's probably many bugs, so be aware. Also, I have no idea if it'll work on any other archives - it only supports the very strict PKZip structure used in the official res files, and may totally crash when working with personal builds or random zip files. It only supports Brotli (using a read-only rebuild of BrotliSharpLib by master131) and Uncompressed files. I will not be adding write support. Ever.
Not guaranteed to work with past or future versions of Pale Moon, personal builds, or other UXP applications.