If you didn't support any other browser, there by definition was no extra burden. In many enterprise shops you had IE and nothing else to worry about. I myself worked on apps where we only tested on IE because that's all the client used.

You'd have been working half-days if the only browser you supported was anything other than MSIE6.

In fact this isn't true, because one thing that IE did better than any other browser was scriptable async http calls and XML transformation with XSLT in the browser. At the time no other browser could do that. We were using javascript to get XML data from the server using the ActiveX XMLHTTP object, and transforming it client side with XSLT to generate HTML. This enabled filtering, sorting, and different presentations of data all client-side. This was hugely productive compared to imperative-style Response.Write HTML building, and at the time was only possible in IE.