It is possible to easily connect to the proxy server from the Firefox browser. While this is possible on a desktop computer, it is not from the Firefox mobile app.
Steps

Step 1. Run Firefox
Firefox's icon is a blue globe with a red fox at the bottom.

Step 2. Click ☰
This icon is in the upper right corner of the window. A menu will then appear.

Step 3. Click Options
The mention is visible in the middle of the drop-down menu. Selecting it takes you to the settings page.
- With a computer running macOS or Linux, click Preferences.

Step 4. Click General
This tab, a cogwheel, is the first in the left column.
- If this tab is in blue, the page General is activated.

Step 5. Scroll to the bottom
There is the section Network settings.

Step 6. Click Settings
To the right of the line just below the heading name Network settings is a button Settings. Click on it to open the connection settings window.

Step 7. Select the Manual proxy configuration check box
This is the fourth in the list, below are several fields visible.

Step 8. Enter the proxy server information
You must fill in at least the following fields:
- HTTP proxy: you will type the IP address of the proxy server;
- Harbor: located on the same line, type the reference of the server access port.

Step 9. Select the Also use this proxy for FTP and HTTPS check box
This mention is found just below the fields you have just completed.

Step 10. Click OK
The button is on the bottom right. In doing so, you save the new settings.
Advice
- If your company connects through a proxy server, your manager will give you the server's IP address and port.
- On some versions of Firefox, the part reserved for the proxy server is in the advanced settings section, then in a section Network.