Disabling Extension Compatibility Checking in Firefox 3.0

*Note: Installing the above extension or accomplishing the "Disable compatibility check" procedure as described below does not mean that all your desired extensions will actually work. It only means that you’ll be able to install them. If you find that an incompatible extension renders Firefox 3.0 unusable then just open Firefox in "Safe Mode", select "Disable all extensions" and you should be able to uninstall the offending extension and then restart Firefox normally.

Manually disabling extension compatibility checking in Firefox 3.0:

Here’s what you do: (don’t include the quotation marks I’ve placed around the texts and statements when typing them in)

1. In your address bar, type "about:config" (remember, no quotes). You’ll receive a warning box that says anything you do here might void your warranty. Click the button that says you’ll promise to behave.
2. Once the configuration page loads, right click anywhere within the configuration options list and select "New" and then select "Boolean" from the 3 choices given (Boolean is a "true" or "false" type statement for those that don’t know).
3. The next box that pops will ask you to "Enter the preference name". Enter the following statement: "extensions.checkCompatibility".
4. Another box will pop up asking you to choose the Boolean value. Select "False" and click OK. You’re new statement should now appear in the list of configuration options.
5. Restart your browser and you can try out those extensions you’ve been wanting to install.

Yet another Note: If you entered the wrong type (like String instead of Boolean) or perhaps misspelled the statement, your new entry will most likely be removed from "about.config" when the browser is restarted giving you another chance to try again. If the bogus entry isn’t removed then right-clik on the entry and select "Reset" then restart the browser once more. The entry will be gone.

Once you make this change you’ll find that the "Add-ons Manager" will display a drop-down yellow bar that warns you that compatibility checking has been disabled and provides you an "Enable" button as shown below:

Again, overriding extension compatibility does not guarantee that an incompatible plugin will work. You may find that an extension that’s not yet updated can work properly, cause Firefox to immediately fail, seriously degrade performance or subtly degrade it over time. I do this myself but 95% of the time it’s for testing purposes only. You take your chances when you disable a safety feature like this.

Be that as it may, welcome to the world of hacking Firefox.

补充:还有一个笨办法,我一直用.呵呵.
手工下载扩展的安装包，比如 a.xpi，打开方式选择winrar，打开它，释放出其中的 install.rdf 文件，然后用文本编辑器打开 install.rdf，手工修改文件内容。
找到 em:maxVersion 标签，把类似 2.0.0.* 的内容改成 3.*。
然后，利用winrar再把 install.rdf 文件添加进去，把原来的文件替换掉，保存退出。
之后，用新的 a.xpi 文件在firefix中打开即可安装，不会再产生版本不兼容的麻烦了，我试过了 gladder YSlow 均没问题。