Find <string> and replace with <same string><text> in Firefox hangs which resultantly blanks out page once published

Issue Summary

In Firefox, using the Confluence Editor Find and Replace of say "30" with "30th" causes the Editor to hang.

Eventually, Firefox allows you to Stop the long loading page and then writes:

30thththththththth...........

to the page.

Environment

Specific to Firefox.

Steps to Reproduce

1. Use Firefox (reproducible on Firefox v67 or v68).
2. Create a page that contains the text "30".
3. Use the Confluence built-in find/replace feature to find "30" and replace it with "30th".
4. Click the "Replace all" button.
5. The browser/page will hang for a while.
6. Firefox will display a yellow banner "A web page is slowing down your browser. What would you like to do?" with options "Stop It" or "Wait".
7. Click "Stop It"
8. The page now has a line that looks like "30thththththththththththth....."
9. Continue to edit the page (delete the extra th's and add a new line)
10. Click the "Update" button to save your changes
11. You'll now be greeted with your saved page but it will be blank.
12. You can restore your previous version, but the edits you made will have been lost.

Expected Results

Find and replace should find the matching text "30" and replace all occurrences with "30th".

Actual Results

1. The page now has a line that looks like "30thththththththththththth....."
2. Continue to edit the page (delete the extra th's and add a new line)
3. Click the "Update" button to save your changes
4. You'll now be greeted with your saved page but it will be blank.
5. You can restore your previous version, but the edits you made will have been lost.

Other search combinations does work in Firefox, this appears to be specific with Confluence Find <number>, replace <same number><text> in Firefox

e.g.

Confluence Find "30" with "30th" does not work in Firefox
Confluence Find "test" with "testing" does not work in Firefox
Confluence Find "30" with "40th" does work in Firefox
Confluence Find "world" with "galaxy" does work in Firefox

Workaround

The Confluence Find and Replace of a <string> with the <same string><text> works as expected in Chrome, Safari and IE11