Regarding donating - keep in mind that this theme is for both Firefox and Thunderbird. So, for someone like me who uses it for both, $5 is a reasonable price. If you use it only for one or the other - consider donating $2.50.
I do understand, though, the tight budget comment. I have been unemployed for almost a year and can say that even though I wish to donate to a worthy effort such as this, it is not high on who needs my money to survive list.
I have been unemployed for almost a year now so I understand the budgetary constraints mentioned in some of the posts. As Mandy pointed out, the donation amount is a suggestion - donate what you are able and let the developer know (there is a text box to add comments with your donations). Believe me, he will appreciate any amount that is donated.
Remember, too, that your donation is one sign of support and interest in this theme. And yes, after making this speech I am finally going to make a donation even though my financial situation is bleak a the moment.