If Mozilla Firefox is becoming unresponsive when you type or open a new page it could be an issue with the software.
Try to change these settings and see if it fixes your slow typing issue.
1. Open Firefox.
2. Type about:config in the web address field. You will need to agree to "I'll be careful, I promise!" to get into the config file.
3. Once in the config file type this in the search box. layers.offmainthreadcomposition.enabled
4. Now rt click on layers.offmainthreadcomposition.enabled and choose toggle to change from true to false.
5. Close Firefox and reopen it. The slow typing should be resolved.