I see questions asked frequently about FireFox rendering problems in Asp.Net forums.

What is "rendering engine"?

A rendering engine, also known as a layout engine, is the code that tells the browser how to display web content and available style information in the browser window

Here are some options to solve those:

1. Always check  rendered web pages for XHTML compatibility using the W3C tests at   http://validator.w3.org . Most issues will fixed by fixing all non standards compliant HTML.
2. Set clientTarget="UpLevel" in Page Directive. This will override automatic detection of browser capabilities and specifies how a page renders for particular browser clients.
3. Here is link to a guide to make your page as accessible as possible.   http://www.anybrowser.org/campaign/abdesign.html