Download and configure an old version of Firefox and Flash using a different profile, so you can access old Flash content isolated from your main profile.

setup-firefox-flash-linux.sh

#!/bin/bash
FIREFOX_PATH="$HOME/.local/opt/firefox84"
FIREFOX_PROFILE="$HOME/.mozilla/firefox/flash-profile"

[ ! -f "$FIREFOX_PATH/firefox/firefox-bin" ] && (
    echo "Downloading Firefox 84"
    mkdir -p "$FIREFOX_PATH"
    curl -L 'https://ftp.mozilla.org/pub/firefox/releases/84.0.2/linux-x86_64/en-US/firefox-84.0.2.tar.bz2' | tar xj -C "$FIREFOX_PATH"
)

[ ! -f "$HOME/.mozilla/plugins/libflashplayer.so" ] && (
    echo "Downloading Flash"
    mkdir -p "$HOME/.mozilla/plugins"
    curl -L 'https://archive.org/download/flashplayerarchivedversions2/352/fp_32.0.0.293_archive.zip/32_0_r0_293%2Fflashplayer32_0r0_293_linux.x86_64.tar.gz' | tar xz -C "$HOME/.mozilla/plugins"
)

mkdir -p "$FIREFOX_PROFILE"

cat <<EOF > "$FIREFOX_PROFILE/user.js"
user_pref("app.update.auto", false);
user_pref("extensions.update.enabled", false);
user_pref("extensions.systemAddon.update.enabled", false);
user_pref("app.update.interval", 0);
user_pref("app.update.checkInstallTime", false);
user_pref("app.update.download.promptMaxAttempts", 0);
user_pref("app.update.elevation.promptMaxAttempts", 0);
user_pref("browser.shell.checkDefaultBrowser", false);
user_pref("services.sync.declinedEngines", "");
user_pref("services.sync.globalScore", 0);
user_pref("services.sync.nextSync", 0);
user_pref("services.sync.engine.tabs.filteredUrls", ".+");
EOF

cat <<EOF > "$FIREFOX_PATH/firefox-flash.sh"
#!/bin/sh
exec "$FIREFOX_PATH/firefox/firefox" --profile "$FIREFOX_PROFILE"
EOF

chmod +x "$FIREFOX_PATH/firefox-flash.sh"

mkdir -p "$HOME/.local/share/applications/"
cat <<-EOF > "$HOME/.local/share/applications/firefox-flash.desktop"
[Desktop Entry]
Version=1.0
Name=Firefox84 + Flash
Exec=$FIREFOX_PATH/firefox-flash.sh
StartupNotify=true
Terminal=false
Icon=$FIREFOX_PATH/firefox/browser/chrome/icons/default/default128.png
Type=Application
Categories=Network;WebBrowser;
EOF

echo "All done!"