Site can not be loaded & stuck in loop

[AdKiller]

When I visit the following sites with HTTPZ addon enabled:

allergologia.hsanmartino.it

www.ospedalesanmartino.it/servizi-alla-persona/informazioni-su-prelievi-ed-esami/laboratorio-di-analisi.html

**EDIT:**
It is a lot more than these 2 sites. It seems to have something to do with speed of loading. Slow sites are more likely to get stuck with this error.

I get an HTTPZ error page with NS_ERROR_CONNECTION_REFUSED message, and Proceed over HTTP & Retry buttons--clicking both results in the same error page. So the website remains inaccessible.

I can visit the above mentioned sites if I do the following:

1. Disable HTTPZ
2. Load the site
3. Enable HTTPZ
4. Refresh the site
5. It will work from that point on and there is no error page (HTTP)

Firefox 72.0.2
HTTPZ 0.12.0

[AdKiller]

It is a lot more than these 2 sites. It seems to have something to do with speed of loading. Slow sites are more likely to get stuck with this error.

[grahamperrin]

http://allergologia.hsanmartino.it/ is not found at the time of writing.

http://www.ospedalesanmartino.it/servizi-alla-persona/informazioni-su-prelievi-ed-esami/laboratorio-di-analisi.html:

• with HTTPZ redirected to
  https://www.ospedalesanmartino.it/servizi-alla-persona/informazioni-su-prelievi-ed-esami/laboratorio-di-analisi.html immediately without difficulty
• in a different Firefox profile without HTTPZ (all extensions disabled) redirects to https://www.ospedalesanmartino.it/servizi-alla-persona/informazioni-su-prelievi-ed-esami/laboratorio-di-analisi.html however the redirect took much longer.