You select contacts just like any other item you want to select. Click the first one, move down 25 contacts, hold the Shift key and click that contact. Now that block of 25 is selected.

This page describes Windows shortcut for multiple selections. http://windows.microsoft.com/en-us/windows/select-multiple-files-folders#1TC=windows-7

Thunderbird itself limits to about 60 addressees. The 100 is not necessarily the most relevant limitation to overcome.

You could use a Mailing List and Bcc, or Mail Merge, but I suspect either of these will get you into conflict with NightmareHost. Or use a service such as Mailchimp which is specifically designed for mass mailings.