I have been having a problem when I am on Tumblr. Sometimes Firefox blocks the next screen. How do I stop this?

When I log out, Firefox stops that screen from opening. I get this error message: "Firefox prevented this page from automatically redirecting to another page". How do I get into Firefox to change its settings to stop this? Also, when I tried to install the software that gathers "Troubleshooting Information" I get this message: "Firefox prevented this site (Support.Mozilla.org) from asking you to install software on your computer." "try these manual steps" was useless because I saw no way to open "Help" on this screen.