Fix memory leaks

[metastable-void]

Describe the bug
There is a memory leak related to per-container overrides.

To Reproduce
Steps to reproduce the behavior:

1. Enable Per-container User-Agent Overrides.
2. Browse as normal.
3. Sometimes Firefox becomes unresponsive.

Expected behavior
Operates as normal, causing no memory leaks.

Screenshots

Console outputs
If applicable, open about:debugging > This Firefox and select Inspect for this extension. Then copy and paste what is in the console (you can hide your private information, if any).

Nothing interesting.

Firefox (please complete the following information):

• OS: macOS, arm64
• Browser edition and version: release 111.0
• Extension version [e.g. 12] 11.4.0

Additional context
https://discourse.mozilla.org/t/container-tab-groups-memory-leak-with-user-agent-overrides-enabled/111524

[metastable-void]

about memory (live measurement).pdf

Memory mesurement for 11.6.0.

[metastable-void]

Here no visible memory leaks are observed.

[metastable-void]

For a permanent and fundamental solution, we are moving to a nonpersistent background page.

[metastable-void]

Fixed possible sources for memory leaks in language overrides codes.

[gmarkley-VI]

Is this still an issue?

[metastable-void]

@gmarkley-VI Combined with recent updates to Firefox, we do not see major performance issues with the extension, but if you find any, please report it here.

[ttiikeri]

I had major memory issues with Container Tab Groups so that I had to disable it to not have a big chunk of the memory be spent on with just a few tabs. When I had less than 5 tabs open, about 10-20 percent of memory would still be used, and if the tab count would go 8+, the memory usage jumps to 35-50 %.

Haven't still debugged what's the actual reason for memory leaking, but this is the situation.