It has been found that Mozilla has been doing changes in FireFox and from version 55 Selenium IDE and many other old extentions is no longer supported.
as the tools is not developed by Cryptzone we are currently looking in to how to solve this issue.
Cryptzone has found the following workaround for those who currently run Selenium IDE.
Mozilla has released a special version of FireFox that do not have these new features named FireFox ESR
Read more here https://www.mozilla.org/en-US/firefox/organizations/faq/
download the installer from https://www.mozilla.org/en-US/firefox/organizations/all/
Ask your company IT Helpdesk if you need assistant with installation of FireFox ESR
Once you have started FireFox ESR you will be able to run/install Selenium IDE as previously.