Firefox is a free, open source web browser from the Mozilla Foundation popular among web designers and developers.
Edit Source with EditPlus
You can view the HTML page source with EditPlus using one of the following two Firefox extensions:
After installing the extension, just right-click on the background, select Launchy > View Source in EditPlus.
This is a new extension that let's you define the program you want to use (from within your extensions manager after install) and then it adds "View Source with EditPlus" to your right-click context menu.
Both of these programs provide a button which you can add to your toolbar by right-clicking on it and selecting "Customize". Then drag the Launchy or the ViewSourceWith icon to your toolbar.
Firefox extensions retrocompatibility
When upgrading to a new major version of Firefox (for example, from v1.5 to v2.0), some extensions may cease to work, because the author only declared compatibility up to the current Firefox version. If the author ceased the development of his extension and doesn't update it to make it compatible to the new Firefox version, you can attempt to force compatibility using the "Make compatible" function of the Firefox extension Nightly Tester Tools.