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.
--> add info from https://support.mozilla.org/en-US/questions/1305175 <-----
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:
accessibility.typeaheadfind.enabletimeout accessibility.typeaheadfind.timeout mail.server.server2.timeout mail.server.server4.timeout mailnews.tcptimeout network.ftp.idleConnectionTimeout network.http.keep-alive.timeout network.proxy.failover_timeout 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.
Settings using the default value are not stored in prefs.js, and will be automatically deleted if you add them by editing the file. This means that not every setting may be listed in the config editor.
Normally this doesn't cause a problem but it means that if you use multiple versions of Thunderbird with the same profile a modified setting could get automatically deleted. This might occur while testing, or if you ran into some bugs with the current release and experimented with installing an older version in a different directory to see if it worked around the problems.
For example, extensions.strictCompatibility has always existed, defaulting to false. In version 60 its default setting was changed to true. If while running version 60.0 you set it to false, exit and run version 52.9.1 it is now using the default value, so extensions.strictCompatibility is automatically deleted from prefs.js. If you run version 60.0 again the setting becomes true (the new default value, since its not defined in prefs.js anymore). There is no warning when this occurs. The same issue also occurred with the hardware acceleration settings (gfx.direct2d.disabled, layers.prefer-opengl , and layers.acceleration.disabled), due to the developers changing the default setting several times. So far those appear to be the only settings where this is an issue.
Normally you don't want to use user.js but if you are concerned that a setting might change behind your back the solution is to define it in user.js.