Sage notebook only runs when a browser is already open

edit

I just installed sage 8.8 (it's not the newest sage, but it works for the most part) on my Linux machine and I had everything working at some point but the other day it isn't working as I'd expect it to be. When I do

$ sage -n jupyter --browser=firefox

in the terminal without Firefox opened, I get

┌────────────────────────────────────────────────────────────────────┐
│ SageMath version 8.8, Release Date: 2019-06-26                     │
│ Using Python 2.7.15. Type "help()" for help.                       │
└────────────────────────────────────────────────────────────────────┘
Please wait while the Sage Jupyter Notebook server starts...
[I 09:42:16.791 NotebookApp] Using MathJax: nbextensions/mathjax/MathJax.js
[I 09:42:17.016 NotebookApp] The port 8888 is already in use, trying another port.
[I 09:42:17.016 NotebookApp] The port 8889 is already in use, trying another port.
[I 09:42:17.017 NotebookApp] The port 8890 is already in use, trying another port.
[I 09:42:17.017 NotebookApp] The port 8891 is already in use, trying another port.
[I 09:42:17.017 NotebookApp] The port 8892 is already in use, trying another port.
[I 09:42:17.023 NotebookApp] Serving notebooks from local directory: /home/user
[I 09:42:17.023 NotebookApp] The Jupyter Notebook is running at:
[I 09:42:17.024 NotebookApp] <local host>
[I 09:42:17.024 NotebookApp] Use Control-C to stop this server and shut down all kernels (twice to skip confirmation).
[C 09:42:17.027 NotebookApp] 

    To access the notebook, open this file in a browser:
        <server file>
    Or copy and paste one of these URLs:
        <localhost>

(I don't have enough karma to actually post the links but "local host" is the link to the notebook and "server file" is the HTML file in "/var/run/user/.../nbserver-...-open.html") which seems fine but Firefox doesn't appear to be open. When I try to open another Firefox browser, I get a message saying that there is already another browser opened when there isn't and I would have to kill the Firefox processes in order to open it again. Interestingly, when I try the same command again with a browser already opened, a new tab with the sage notebook appears.

I get the feeling that the problem could be coming from my init.sage file but I'm not sure if that would be the case.

~/.sage/init.sage:
==================================
from sage.misc.viewer import viewer
viewer.browser('open -a /usr/bin/firefox')

My environment variable SAGE_BROWSER is set to firefox:

$ echo $SAGE_BROWSER 
firefox

If there's anything else that you'd like me to provide, let me know.