JavaScript settings
First of all, MultiZilla makes extensive use of JavaScript. Most of the bugs are caused by a typo, or design error, in a JavaScript source file. This is also why you should check your JavaScript console for possible errors before you file bug reports. However, Mozilla won't display all warnings and errors by default. You need to change some of your preferences to make it display all messages. Here's a screenshot of these settings in the Quick Prefs menu:

Change your settings, when needed, to match the ones on the screenshot. All JavaScript warnings (less important) and errors (important) will be displayed on the JavaScript Console with these settings. Note that Java messages are not displayed on the JS console, because Java has its own console (remember, Java is not Javascript!).

Launch Error Console
There are two ways to launch the Error Console in SeaMonkey with MultiZilla:

• Select Tools → Web Development → Error console from the menu.
• Click on this button on the MultiZilla toolbar:
• 

JavaScript Errors
JavaScript Errors will prevent MultiZilla, or any other application, from working as expected. Here is a screenshot of the Error Console with two sets of errors:

The first two errors in this screenshot are caused by a problem on line 331 in the file multiviewsTabs.js It's part of the MultiZilla source, because it starts with chrome://multiviews/ We are only interested in warnings and errors that start with chrome: Here are some examples for you:
chrome://multiviews/
chrome://quickprefs/
chrome://googlebox/

The last two errors on the screenshot are not part of the MultiZilla source code. However, these errors can still be caused by a third party add-on, so we might need them to fix the bug.

JavaScript Warnings
JavaScript Warnings are less harmful but we like to fix all possible JavaScript Errors/Warnings. Search the MultiZilla bug database (Bugzilla) and file a new bug in case you can't find a duplicate.

Bug with JavaScript error
Okay, say you've launced the Error Console and found a Warning/Error in the MultiZilla source code, or you think it is. Now, search in the MultiZilla bug database (bugzilla) and file a new bug in case you can't find a duplicate for it. Copy the text from the console to the bug report and add steps to reproduce.

Bug without JavaScript errors
Not all type of bugs display JavaScript Warnings/Errors on the JavaScript console. There are more ways to trigger a bug, like invalid or incorrect XBL bindings. Also, take a look at the next screenshot, that might be another way to 'hide' JavaScript errors and warnings.

JS Protocol Filter
MultiZilla adds a protocol filter to the JavaScript Console. Clicking the Options button and then selecting the Filter tab makes it possible for you to filter all unwanted protocols out of the list. In short, make sure to tick Block from Web and Block from File when tracking SeaMonkey/MultiZilla errors.

Right-click on a link in the JavaScript Console to open the source in a new tab, a new tab in a new window, but you can also use ctrl-click or middle-click, or the default way of view source. Here's a screenshot of the modified context menu

Note: The new features were introduced in a prior version of MultiZilla and are very useful for web-developers/designers and bugtrackers, but it was already being used before thatby HJ and Neil. You might consider reading this Mozilla bug report where you can see how old this thing really is.

Cascade Style Sheets errors
We use several Cascade Style Sheets (CSS) for MultiZilla. These files can cause bugs that won't produce warnings or errors on the JavaScript console but make a mess of your browser if something goes wrong.

XUL errors
SeaMonkey is constantly changing its XUL code so we have to keep up with some of these updates. This can cause serious problems. Make sure to test your bug with the most recent MultiZilla version available for the installed SeaMonkey version. We won't fix bugs for older SeaMonkey versions; you simply have to update your SeaMonkey build in case you run into this kind of troubles.

Debug Console
SeaMonkey has a build in debug console. You can start SeaMonkey with: seamonkey -console to launch it. It's very useful in case SeaMonkey doesn't startup, especially after installing third party add-ons/themes. Check if you see chrome: related errors on the console. This information might help us to fix bugs even quicker.

Third party theme support
MultiZilla is designed for the two default mozilla themes, Classic and Modern. We don't support other themes. You should always test your bug with the default Classic and Modern themes. We're not going to install all available themes. Theme errors must be fixed by theme authors because we don't do that.

New profile
There are several ways to run into profile errors with SeaMonkey and MultiZilla will also suffer from this kind of problems. You should always try a new profile, before you file a bug report.

Start SeaMonkey with: seamonkey -profilemanager and select Create Profile. And check if that helps.

MultiZilla bug?
It is a good habit to check if the bug is caused by SeaMonkey, MultiZilla, or the combination of the two. You must also disable other installed extensions/add-ons, because they can also cause problems.