There’s a bug open for Firefox and you can change VSCode’s behavior by putting this in your keybindings.json:
{
"key": "shift+insert",
"command": "editor.action.selectionClipboardPaste",
"when": "textInputFocus && !editorReadonly"
}
Not sure why either of them are messing with a “standard” shortcut…
If you want a more system-wide solution there are utilities that let you sync the PRIMARY (on selection) and CLIPBOARD (Ctrl+c) buffers mentioned in the Arch Wiki entry.
Looked around some more - maybe it’s because they’re all GTK apps?
https://unix.stackexchange.com/questions/178070/why-does-shiftinsert-paste-from-clipboard-in-some-applications-and-primary-in-o