Tips and Tricks

Debugging
Some general tips to speed up your development workflow:
To debug code running in the browser context (e.g.: your Experiment APIs) you must use the global browser console (Ctrl+Shift+J) or developer toolbox (Ctrl+Shift+I). 

To debug code running in a content page of your extension (e.g.: your background script), you need to select "debug add-ons" from the gear icon in the add-on tab and then inspect your add-on. 

Hint: Alternatively you can enable content messages in the global browser console or in the developer toolbox as well. This will also show console output from popus (e.g. from browser_action):
The add-on debugging tools accessible through the add-on page's gear icon permit to directly install add-ons without packaging them. Using that option permits to reload the add-on without restarting Thunderbird.
There is also a debug guide from the extension workshop with more details and the most recent information on debugging. It is written for Mozilla Firefox but applies for Thunderbird as well.
Parallel hosting of legacy add-ons and MailExtensions on addons.thunderbird.net (ATN)
After updating your add-on for Thunderbird 68 and beyond, it will be treated as a WebExtension. While uploading it to addons.thunderbird.net (ATN), you will see a warning, that you cannot upload a new legacy version of your add-on afterwards. However:
It is possible to maintain a legacy version and a WebExtension version of your add-on in parallel on ATN! You just need to use a higher major version number for the WebExtension version of your add-on and keep the old major version number when releasing a new legacy version. Basically releasing them in two different branches.
Let's assume the last published legacy version of your add-on for Thunderbird 60 is 2.6. Upload the new WebExtension version for Thunderbird 68 as 3.0 and increase the version number with each new release of the WebExtension as usual. Every time you update the legacy version for Thunderbird 60, pick a version number from the 2.x branch.
Forcing modified XUL files and JavaScript to reload
One of the stumbling blocks that add-on developers frequently encounter is Thunderbird's caching mechanisms, which cache XUL and JavaScript files for performance reasons so that they don't have to be re-read from their source and parsed or compiled repeatedly. If you're developing an add-on and iterating through changes to XUL and JavaScript files, you may find that Thunderbird insists on continuing to use old versions even after you've changed them and restarted.
-purgecaches command-line option
DMO claims that specifying the -purgecaches command line option when launching Thunderbird will force it to purge the JavaScript cache.
Invalidate caches programmatically
Executing this JavaScript code inside Thunderbird will cause the XUL cache to be invalidated the next time it restarts:
const { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");​
Services.obs.notifyObservers(null, "startupcache-invalidate", null);
One way to make this happen automatically is by installing the userChromeJS add-on and then putting the code above into the file chrome/userChrome.js within your Thunderbird profile directory.
nglayout.debug.disable_xul_cache preference (TB68)
Go into the advanced preferences editor, create a boolean preference called nglayout.debug.disable_xul_cache if it doesn't already exist, set it to true, and restart Thunderbird.