You need to click on account in the accountsettings pane first and then click the set default option.

I have this same problem. I'm trying to make a gmail account the default email address in Thunderbird and even going by your suggestion to click on the email address name in the left pane, then selecting Account Activity, the "Default" is still greyed out and you can't set it as the default email. Any other suggestions?

I got a notice in an email in my Thunderbird program that this problem had been answered but there was no answer whenever I clicked on it. I still want to make my gmail account the default but no matter what I do, "Set As Default" is greyed out so that you can't select the gmail account as the default. I have clicked on the gmail account, then opened "Account Activities" to select the default, as several have suggested here, but it is still greyed out to where you cant select it as the default.

If I delete the current default account forcing the remainig gmail account to become the default, will I lose all of my emails under the original default account?

Okay, for those of you who have had this same problem, I discovered how to "ungrey" the "Set As Default" selection. Go to your default account, click on it, select "Account Actions" and "Set As Default" is NOT greyed out. Click on it. Then go to the account you're trying to set as the default and go through the same process as your original account and the "Set As Default" is now NOT greyed out and you can click on it to select this account as the default.

I know that sounds crazy, but it works.