Many of these need to be fixed with the extension converter before SeaMonkey can install them. If SeaMonkey doesn’t allow you to install them, download them instead and then upload them to the SeaMonkey Extension Converter.
From addons.thunderbird.net
- PDF Viewer
- Dark Background and Light Text
- Dorando keyconfig
- SeaMonkey Vertical Tab Bar
- SeaTab-X-2
- TimerFox
PaleMoon addons
Stylem is like Stylish or Stylus, except better because it has a “check for errors” option right there in the editor. I wish I’d known about this sooner; I had a lot of cruft to clear out of my userContent.css file.