'Find' option in Editor only finds first instance of text when using new version of Firefox browser

Summary

'Find' option in Editor only finds first instance of text when using new version of Firefox browser.

Environment

• Firefox Quantum 63.0.3
• Tested in Confluence 6.6.3, Confluence 6.6.9 and Confluence 6.12.1
• Also tested in different OS (Mac OSX Mojave and CentOS 7)

Steps to Reproduce

1. In Confluence, create a new page and insert multiple times the same word, for example: 'Content'.
   
2. In Edit mode, hit ctrl+F, cmmd+F, or click the 'Find/Replace' icon on the top right corner.
   
3. In the 'Find' box type the written word 'Content' (or whichever word used) and hit 'enter' or click 'Next'.
   
4. After finding the first instance of the search, click 'Next' again.
   
   Notice that nothing changes and it doesn't highlight the next word.

Expected Results

It is expected to see the next instance of the word highlighted in the same way we see the first instance.
Exemplo screenshots taken from Google Chrome:

1. Type 'Content' in the 'Find' box and click 'Next'
   
2. Click 'Next' highlight moves to next word
   
3. Click 'Next' highlight moves to next word, so on and so on...
   

Actual Results

Only the first instance of the word is highlighted even after clicking 'Next'

Workaround

• Use older versions of Firefox, version 62.0.3 works fine
• Use different browsers, tested with Safari and Google Chrome and it works fine.