I guess I don't stay up to date like I should; when I clicked to install this extension, it first told me my Firefox was out of date. So, I installed Firefox 1.5. Somehow, I don't remember how, the setting for software installs was turned off in version 1.0. There is no easy way to turn it back on in 1.5. I finally found a reference in a bug file, went to the raw configuration file (about:config), found the referenced value and set it to "true". Now, I could finally install the image magnifier.
That worked, but it showed me a list of installed extensions, one of which was Spellcheck. However, it now stated that Spellcheck (which I really liked) was disable because it wasn't compatible with Firefox 1.5.
So, I gained an image magnifier (which I may or may not use) and lost my Spellcheck. I'm not sure I gained anything...