how do I block less2pay extension

Did you already try disabling or removing the extension on the Add-ons page? Either: * Ctrl+Shift+a * "3-bar" menu button (or Tools menu) > Add-ons In the left column, click Extensions. Then, if in doubt, disable. Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step. If it isn't listed or keeps coming back to life, there may be external software that needs to be cleaned out. Let us know what you find.