Hi JMiller123,
I have searched, and also just found documents about TinyMCE with SharePoint 2007, there is no article about TinyMCE with SharePoint Online.
For this issue, I also suggest you use IE for your SharePoint.
If you still want to use Chrome or Firefox for SharePoint, you can try to install IE Tab extension for Chrome or Firefox, then you can use IE in Chrome or Firefox.
https://www.ietab.net/
https://addons.mozilla.org/en-US/firefox/addon/ie-tab/
While Microsoft supports and encourages open-source software development, it does not provide technical support for the softwares from third parties, nor does it guarantee that the software will work as described.
Thanks,
Wendy
TechNet Community Support
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