Which version of Firefox and which OS are you using? I've not had trouble loading RootsSearch. I was using Firefox v67.0.2 (64-bit) on Win10 and it loaded fine. I upgraded to v68.0 (which was a pending update) and it still loaded fine. Then I upgraded to v68.0.1 (Firefox now says I'm finally up-to-date!) and it still loads fine.
Other things to consider: have you recently updated your OS or added/updated any browser add-ons/extensions or added/updated any security/privacy software that might be causing the problem?