How to install an extension on Firefox while creating a webdriver through Robotframework

I want to start a Firefox session with an installed extension: https://addons.mozilla.org/en-US/firefox/addon/har-export-trigger/

My environment:

robotframework 3.2a1 
robotframework-seleniumlibrary 4.0.0a2
selenium 3.141.0
firefox 72.0
geckodriver 0.26.0

Here is my script:

${browser_options}  Evaluate  sys.modules['selenium.webdriver'].FirefoxProfile()  sys, selenium.webdriver

call method     ${browser_options}   add_extension  ${EXECDIR}/har_export_trigger-0.6.1-an+fx.xpi

create webdriver   Firefox   firefox_profile=${browser_options}

As a result, the extension is not installed (the steps just pass with no errors).

Any suggestions?

Thanks in advance!