Sandboxie 5.33.3 and Firefox problem. Help please.

Hi!

Having problems with sandboxed Firefox.

So here's my configuration:

Windows 10 Pro x64 v1909 (18363.719)
Sandboxie 5.33.3 (tried also with 5.33.1 since that version worked on my other laptop with the same Kaspersky product/version and same Firefox versions installed and the same Windows 10 version, as does the new one, 5.33.3)
Kaspersky Internet Security 20.0.14.1085 (i)
Firefox 74.0 (64-bit) - installed with following add-ons: "Adobe Acrobat", "Dark Background and Light Text", Disable WebRTC", "Firefox Multi-Account Containers", "HTTPS Everywhere", "Kaspersky Protection", "Privacy Badger", "uBlock Origin"
Firefox 74.0 (64-bit) - portable, started through portableapps "launcher" exe, no add-ons installed, proxy activated
Opera 67.0.3575.79 - portable, no add-ons installed
Chrome 80.0.3987.132 (32 bit) - portable, started through portableapps "launcher" exe, no add-ons installed

Problems:

1. Problems when Firefox portable started in Deafult sandbox (right click on exe - "run sandboxed"), deleted contents right before starting. Firefox appears to start normally (with uac prompt since started through portableapps "launcher"), no errors pop up, opens web pages normally, downloads and recovery work normally, but... copy-pasting "inside" of sandbox (from one firefox tab address bar to another) and copy in sandbox - paste outside sandbox don't work. On right click "paste" is greyed, ctrl+v yields no result. Copying from outside and pasting inside sandbox - works normally. Copy-pasting in notepad++, for instance, works as expected - both in and outside of sandbox and in "every direction".

2. Opera portable, started in same manner works as expected. (tested copy-paste in every direction, opening web pages, downloading+recovery)

3. Chrome portable, started in same manner works as expected. (tested copy-paste in every direction, opening web pages, downloading+recovery)

4. Problems when Firefox (installed) started by right-click on exe - "start sandboxed".

- Error pops up. SBIE2335 Initialization failed for process plugins-setup.exe [33 / 0]
- Firefox window opens
- After a couple of seconds an UAC prompt "Sandboxie Start", then the error pops up once again. SBIE2335 Initialization failed for process plugins-setup.exe [33 / 0]
- After another couple of seconds a windows error dialogue - title: "Windows - Application Error" with "The application was unable to start correctly (0xc0000022). Click OK to close the application.
- After dismissing alerts both from Sandboxie and the "WIndows - Application Error" message, the browser continues to behave just like the portable version. Opens web pages, downloads/recovery works; no copy-pasting inside sandbox, just as portable version.
- After closing Firefox, "SandboxieRpcSc.exe" and "WCChromeNativeMessageingHost.exe" remain running for approx 30 seconds

Same thing happens both when Kaspersky active, "paused" and stopped. Sandbox contents deleted before every run. Interetingly though, no errors pop up when Firefox (installed) started with add-ons disabled (kind of what I expected since the process failing is called "plugins-setup.exe"), but copy-pasting has the same problem as described above. Tried enabling one add-on at a time, turns out the errors about "plugins-setup.exe" and the windows error (and the UAC prompt with installed Firefox) pop up only when Kaspersky add-on is enabled. Unfortunately, nothing I did helped with the copy-paste problem. Firefox (installed) works normally with Kaspersky add-on active when not sandboxed.

After trying with and without the Kaspersky add-on in Firefox (installed), tried the same with Chrome (portable) - the same error as with Firefox exhibits when Kaspersky add-on active, but copy-paste works.

Unfortunately, Firefox is the only browser I know of that supports a "per-browser" proxy setting and I really need that. Please tell me if I can give you any more info to aid you in helping me.

Thanks in advance.