Here's what was ported across from the Chrome version into the Firefox extension — all 3 files touched:
scheduler.js — biggest change:

Storage is now async (loadQueue/saveQueue return promises), with chrome.storage.local tried first and localStorage as fallback — same pattern as Chrome, works in both environments
add(), remove(), getQueue() are all async now
tick() + _fireNext() replaced by _fireAll() — fires the entire queue sequentially with a 15-second gap between prompts (BETWEEN_DELAY_MS) instead of sending one and stopping
New testSend(text) method that the "Send now (test)" button calls
Timer is now armed directly in updateUsage() instead of relying on tick() polling

ui.js — scheduler panel:

"Queue Prompt" button renamed to "Queue for reset" and moved into a flex btnRow
New "Send now (test)" button alongside it — calls testSend, shows success/error feedback inline
New _feedback div with green/red states
_addPrompt, _renderQueue, delete handlers all made async to await the new async storage
Queue items now show a numbered order badge (①②③…) so you can see firing sequence
Prompt truncation tightened from 90 → 80 chars

main.js — copy button now properly injected:

The attachUsageLine patch now also appends ui.copyButton (with its separator dot) into usageMetaGroup — the copy button was wired up in ui.js all along but was never being attached to the DOM in the FF version