Its risky to use an add-on with Thunderbird to make it a general purpose web browser as it was not designed for that, and the developers have a long history of deliberately delaying adopting security fixes that are browser-centric. Thunderbird is also very prone to regression errors and has far less testing than Firefox.
I recommend you only use add-ons such as BrowseInTab with a small number of web sites that you know are safe.
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despite a bag of money mozillas is sitting one, TB's leadership shown it's remarkable incompetence by creating a runaway train of bugs and anti-features."
Its misleading to still think of Thunderbird as part of Mozilla. They've been independent for a good while. Mozilla stopped funding the project but still acts as a fiscal/legal home. The separation started in 2012 when Mitchell Baker publicly stated Mozilla's decision to transition Thunderbird to a new release and governance model.
In 2017 the Thunderbird project decided between Software Freedom Conservancy (SFC), the Document Foundation (TDF), or a new deal with Mozilla Foundation as their new home. They decided to make a new deal with Mozilla. They now use a new subsidiary of Mozilla, MZLA Technologies Corporation, as their home to provide them more freedom in what products and services they offer. The Thunderbird Council has confirmed in the tb-planning mailing list their earlier statement that if their needs change Mozilla will let them chose a different home again (not affiliated with Mozilla).
The project is not funded by Mozilla, it relies upon user donations for its income. See
https://blog.thunderbird.net/2022/05/th ... al-report/ . Firefox developers have been told not to waste any time preventing any changes from creating bugs in Thunderbird. This is the so-called "Thunderbird tax" that Mozilla management in 2015 very publicly complained was effecting Firefox development.
https://arstechnica.com/information-tec ... o-drop-it/
Mozilla still owns the Thunderbird trademarks. The project has problems making their mind up about what branding they will use. In some places they still mention Mozilla, in others they've dropped mention of it. There is a bug report about that.
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Which brings me back to a question what options I have, if TB folds in 3-5 years from now completely."
I don't see any reason to worry about that. However, there are other projects based on modifications to Thunderbird's source code such as BetterBird at
https://www.betterbird.eu that might become more mainstream if that happens. BetterBird is lead by a former Thunderbird developer and feeds their fixes to Thunderbird. Some get quietly adopted, others ignored.