If it was a 1 off payment I'd be more inclined. I'm not a big fan of /month payments. But I'm guessing a lot of the features they come out with I'll already have installed and paid for, such as VPN etc.
However, it is good to see Mozilla are looking to create a more sustainable business model with consistent revenue rather than selling search data and advertisements. So if they adjust the price and feature set to be somewhat decent/stuff I have need for and dont currently have then I'd be happy to support them monetarily.