Sorry to hear about your issue. Don’t panic, Firefox is not slow by nature, on the contrary. In order to find out if the issue is related to add-ons, start Firefox in Safe Mode by holding down Shift for a number of seconds when clicking the startup icon or shortcut. Does it make a difference? If so, restart in normal mode, disable all add-ons and enable them one-by-one to see when the issues returns. If this doesn’t help, could you check your proxy settings in Options > Advanced > Network > Settings...? It may be set to use the System proxy settings and cause issues. Please set it to None and post back your results.