Modify Thunderbird settings
From MozillaZine Knowledge Base
The recommended way to add or modify a setting which doesn't have its own input box, checkbox or radio button in said dialogs is to use Tools → Options → Advanced → General → Config Editor, equivalent to Firefox's about:config. You can search for any preference using the filter field, and then double click on a preference to modify it. You can also add settings by right clicking anywhere in the list, selecting new from the context menu, and then select the type (string, integer or boolean), and enter the name of the setting and its value.
You have a problem downloading a large message, or the webmail extension doesn't fetch all of your new mail. Increasing the timeout setting from the default 60 seconds to 120 seconds might help. However, the timeout setting, as with many preferences, cannot be set using the GUI. The first step is to figure out the name of the preference. If you can't find the preference in the knowledge base, try using the Config Editor to identify the name of the preference. If you type timeout in the filter you may see something like the following:
along with their status, type and value. You need a preference whose name begins with mail or mailnews (it tends to have global settings). mail.server.server2.timeout and mail.server.server4.timeout are obviously server specific settings, network.* settings are normally too specialized (NTLM, dialup, cookie related settings etc.) and accessibility.* settings are clearly unrelated. That leaves the mailnews.tcptimeout setting. Double click on it, enter 120, and press the OK button. If you don't want to use the Config Editor you could have used a text editor to add user_pref("mailnews.tcptimeout", 120); to prefs.js instead.
The defaults\pref\mailnews.js file in your Thunderbird program installation directory contains most of the default settings. It uses a pref prefix rather than a user_pref prefix on any settings. For example, pref("mailnews.tcptimeout", 60); . You should not edit that file. However, it's sometimes useful to view that file to find out what preferences exist and what their default values are.
Change connection timeout
If you want to increase the connection timeout and the instructions in the example are too complex:
You can also create an optional user.js file in the same directory as prefs.js. It is mainly used by administrators to set the same settings in several profiles. It is recommended that you don't use it since any settings you add to it will be merged into prefs.js when Thunderbird starts, preventing permanent changes using the Config Editor.