I'd like to help you print your Reconciliation Report, so let's do some more troubleshooting to isolate this behavior.
We can start by using Google Chrome or another web browser besides Firefox since you've tried that. Once you've opened the web browser, open a Private Window or an Incognito Window. Here's how:
Google Chrome: Press Ctrl + Shift + N
Mozilla Firefox: Press Ctrl + Shift + P
Internet Explorer: Press Ctrl + Shift + P
Safari: Press Command + Shift + N
Once you're in the Private Window, log in to your QuickBooks Online account and try printing your Bank Reconciliation Report. If this works, then let's clear your browser's cache. This is to delete temporary internet files and data that are causing unexpected behavior in the product. Here's a link to help you with the process: Clear Web Browser History.
Let me know how this goes for you by leaving me a Reply. I'd be happy to help you further. Have a beautiful day!