If you even occasionally write a blog post, email and an article that need screenshot of a web page attached, you will definitely want to check out Pearl Crescent Page Saver Firefox extension.
Pearl Crescent Page Saver is an extension for Mozilla Firefox that lets you capture images of web pages. These images can be saved in PNG format or (with Firefox 2) in JPEG format. The entire page or just the visible portion may be captured. Options let you control whether images are captured at full size (which is the default) or scaled down to a smaller size. Page Saver uses the canvas feature that was introduced in Firefox 1.5.
I've been using it for a week or two now and it already saved my some time I would spent pressing Alt + Print Screen and then cropping and merging images.
Please note there are two versions. $15 Pro and free Basic. Regular works just fine for me.