I haven't used voice with ChatGPT, but for the typical site, Firefox will prompt you to grant access. If you didn't do that at the time, there might be a notification icon of some type near the lock in the address bar to grant it at your leisure. If not, you can grant permanent access this way:

While you are on a page on the site, call up the Page Info dialog using one of these methods:

• Ctrl+i (for Mac, Command+i)
• (menu bar or Alt+T) Tools > Page Info
• click the lock icon in the address bar, on the drop-down click the > button, then at the bottom click "More Information"

When the dialog comes up, click the Permission icon at the top to show that panel.

Scroll down to "Use the microphone" and uncheck the "Use default" box, and then select "Allow". You can always revoke this later using the Settings page or the customized permissions button next to the lock icon in the address bar.

You may need to reload the page (Ctrl+R) so that ChatGPT queries Firefox again about mic permission.

This is happening for me as well. Voice dictation was working in Firefox as recently as 8-20-25. I followed the instructions above: unchecked chat.openai.com’s “use the microphone,” toggled from “use default” and “always ask” to “allow”, but it still does not work. The microphone functions normally in ElevenLabs inside Firefox. I can replicate this issue on a completely different machine. Voice dictation works on chat.openai.com in Chrome. Did Firefox v142 break something with ChatGPT?

The "dictate" feature seems to work normally on my regular profile (screenshot attached).

But I am using the site at https://chatgpt.com instead of https://chat.openai.com. I have a free account, and if I try your address it redirects me back to the one I usually use.

If you right-click the chat area you are using, does the context menu have a "This Frame" sub-menu? If so, expand that and click Frame Info to access the Page Info dialog of the framed page and see whether setting Allow permission there makes any difference.