New comment.: It is blocking some elements that should be shown. The others blockers did without issues.

Old commentary: There is a problem with the latest version of Firefox 66.0.3 that it unables to download and install extensions. I got around it by downloading the previous version Firefox 66.0.2. Just go to firefox page and choose your system. https://ftp.mozilla.org/pub/firefox/releases/66.0.2/

все ок

unfortunately, Firefox automatically disabled it this morning and it is impossible to download it again: "Download failed. Please check your connection"