Launching jupyter notebook from terminal starts signal desktop messenger as well

I have the same issue here (Ubuntu 18.04) and I discovered that the default application to run *.html-Files was also changed to Signal Desktop App. After changing the default application for *.html backwards to Firefox jupyter notebook launches already in the browser.

I've found a way to do this that is (possibly) more directly related to Jupyter. You can create a Jupyter config file and change the browser preference to whatever you like. Type:

jupyter notebook --generate-config

on the command line. It will tell you where the config file was created (probably $HOME/.jupyter/jupyter_notebook_config.py). Open the config file and look for the following line:

#c.NotebookApp.browser = ''

you can uncomment it and change it to include your desired web browser. For example this is how I changed it to firefox:

c.NotebookApp.browser = '/usr/lib/firefox/firefox'

Save the file, exit and try again.

This worked for me on Ubuntu 16.04.6 (LTS) and I had a similar problem but this answer didn't work for me. The situation I found myself in was that jupyter launched Firefox correctly in a normal environment but it launched the wrong application when I was in a directory that used a different python version which was being controlled via pyenv. I was launching the notebook like this: pipenv run jupyter lab. For completeness it was launching Signal for me too and - no - I don't really understand why. I found help in the thread connected to this issue on the jupyter/notebook Github page.