Your web development is limited just as much by the tools you use, as by your expertize. Imagine trying to build a house with just your bare hands?
It is very possible to do web development with just notepad.exe, and an ftp client. But doing so seriously handicaps you. Here are a few tips to get you going.
Open Source Web Development Tools
You may also want to look at the other web development extensions provided for Firefox.
Aptana is an open source IDE (code editor) built upon Eclipse. It supports Ruby on Rails, Adobe Air, IPhone among other development platforms. Up till now, I haven't found a better Open Source IDE for web developers.
Notepad++ is a drop in replacement for the default notepad.exe on Windows OS. It is also a syntax highlighter for many programming languages and has a built in file manager. This tool offers quick editing of your HTML, CSS, XML etc. if you don't want to waste time loading a full IDE.
Apache, MySQL, PHP on Windows. If you're still doing your web development over FTP, its about time you worked on a local version of your web server. WAMP allows you to easily install an Apache Server, with PHP and MySQL pre-installed.