All the text I type in Thunderbird emails is suddenly underlined in red, as though spelling is wrong - how can I stop this?

Please tell me how to stop this. I have spellcheck enabled and a English (UK) dictionary selected. This problem has just started. My computer is old now and I'm running Windows 10. I had to do a 'hard shut-down' recently when my computer was unresponsive - might this have caused the problem? This underlining is only happening in Thunderbird, not in Word for example. I would be grateful for a simple solution as I'm not very computer-savvy.