If you use Firefox Multi-Account Containers with Firefox sync, sometimes you will get a number of extra containers, even thousands of them. Run this in the devtools console in the Firefox containers section `about:preferences#containers`. Add your own container tags on line 48.

containers.js

let removeContainers = (...containers) => {
    let items, isValid, removeButtons;

    items = document.querySelectorAll("#browserContainersGroupPane richlistitem");
    isValid = (el) => typeof (el) != "undefined" || el != null;
    removeButtons = [];

    items.forEach((el, idx) =>
    {
        const hbox_1 = el.getElementsByTagName("hbox").item(0);
        if (isValid(hbox_1))
        {
            const label = hbox_1.getElementsByTagName("label").item(0);
            if (isValid(label))
            {
                if (label.textContent.slice(0, 3) === "tmp" || containers.includes(label.textContent))
                {
                    let containersContent = "tmp";

                    if (containers.length > 0) {
                        containersContent = `${containers.join(', ')}, or tmp`
                    }

                    console.log(`Element ${idx} is ${containersContent}`);
                    const containerListItem = el.getElementsByClassName("container-buttons").item(0);
                    const buttons = containerListItem.getElementsByTagName("button")

                    Array.prototype.forEach.call(buttons, (button, _bidx) =>
                    {
                        if (button.getAttribute("data-l10n-id") === "containers-remove-button")
                        {
                            removeButtons.push(button.getAttribute("value"));
                        }
                    })
                }
            }
        }
    })

    removeButtons.forEach((button) =>
    {
        let el = document.querySelector(`button[value="${button}"][data-l10n-id="containers-remove-button"]`);
        el.click();
        console.log(`Removed id ${button}`)
    });
}

removeContainers()