Hi! This can happen with any add-ons. It's usually a browser issue, not a problem with the add-on. Please try clearing your browser cache and cookies, restart Firefox, and try installing AdBlock again. If that doesn't work, it may be a conflict with another add-on. Disable all of them and try installing AdBlock again.
If none of that helps, please open a ticket at help.getadblock.com so we can look into this with you. Thanks!
