Helpful info.. (I've been here before because of nonworking TFSC) and my old eyes need help, like a lot of peeps showing up here :)
First, I only use PC Waterfox 64bit 54ver. and I use Thunderbird 32bit 52ver. I have ALL my addon's turned off and not to update including WF and TB. I realize now, That Firefox and Thunderbird will never add some helpful font adjustments for our bad eyes :(
If you read through the reviews. You'll see some info reviews on how to download and install a non expiring version of TFSC on earlier version of FF, Waterfox, TB. I only wanted to help people because I feel their pain on not being able to see and need help and remember to turn off all updates that includes FF, TB, WF. . I still want to give props to the Author of TFSC. Thx you!