One possibility is that the bookmark is for an older address on the site. If you try to access the site using a different method, such as running a web search and going to the site from the search results, does it work any better?

Sometimes error messages about your browser are based on capabilities instead of version numbers. For example, the site may be trying to run a script or use offline storage and it is not working as expected. Along those lines:

Double-check content blockers: Firefox's Tracking Protection feature, and extensions that counter ads and tracking, may break websites that embed third party content (meaning, from a secondary server).

(A) The shield icon toward the left end of the address bar usually turns a bit purplish when content is blocked. Click the icon to learn more or make an exception. See: Enhanced Tracking Protection in Firefox for desktop.

(B) Extensions such as Adblock Plus, Blur, Disconnect, DuckDuckGo Privacy Essentials, Ghostery, NoScript, Privacy Badger, uBlock Origin or uMatrix usually provide a toolbar button to manage blocked content in a page. There may or may not be a number on the icon indicating the number of blocked items; you sometimes need to click the button to see what's going on and test whether you need to make an exception for this site.

Any effect?

I turned it off for the site I was wanting and it seems to work much faster now and no errors.