”Webpage is slowing down your browser” This is the new way the browser tells you there is a problem with the script program(s). https://support.mozilla.org/en-US/kb/warning-unresponsive-script?cache=no

https://support.mozilla.org/en-US/kb/task-manager-tabs-or-extensions-are-slowing-firefox

Are you uploading via the "Browse" button feature like this forum uses to attach a screenshot or is this done otherwise ?

It is possible that (anti-virus) security software is causing the problem.
Try to disable security software temporarily to see if that makes a difference.