You can download and install the Web Developer Toolbar plugin here:
The Web Develoepr Toolbar will appear under the row
of Links in your Firefox browser interface. If for some reason
it does not appear, you can reveal it by selecting View > Toolbars > Web
Below are details on the some of the most helpful menus of the toolbar:
people can and do set their browsers to ignore style sheets, you should
make sure your page is comprehensible without them. Select CSS >
Disable Styles > All Styles to view your page without the style sheet. Reload the page to restore normal view.
The Resize tool lets you see how your page will look on higher and lower screen resolutions. The most common screen resolutions are 800 X 600 and 1024 X 768, so it is good to test both. (e.g. select Resize > 800 X 600). You can add more options by selecting Edit Resize Dimensions. . .