With the extension in the toolbar: my first attempt to use it caused Firefox to stop responding. On FreeBSD, that's extraordinary. After a few minutes I killed the process.

A few months later: no longer using Firefox, version 1.0.0 works fairly well with Waterfox 56.1.0_10. Replaced text in the URLs of 25 tabs in a ~1,260-tab session. The application didn't hang, and the effects of replacement became noticeable reasonably soon (not immediately) after using the extension.